Email Signature Size Guide — Dimensions, Image Sizes & Limits (2026)

Overall signature dimensions

Before we get into the specifics of individual image types, let’s establish the outer bounds of the whole signature block.

Why 600px?

600px became the de facto email standard during the era of 1024px desktop monitors. A typical email client with a sidebar took up about 400px, leaving around 600px for the reading pane. Even in 2026, with wider monitors everywhere, email clients like Outlook, Gmail, and Apple Mail still constrain the message column to roughly 600–700px in their default layouts.

More importantly, mobile email clients scale a 600px email down gracefully. A 700px email, by contrast, requires horizontal scrolling on most phones — a UX failure that makes your signature look unprofessional. The Gmail signature guide and the Outlook signature guide both cover client-specific rendering in more detail.

Why keep height under 200px?

There’s no hard technical limit on height — you could make a 1000px tall signature and most clients would display it. The issue is practical: a signature longer than the email itself is noise. Recipients scroll past it and eventually start filtering people who send them. Short emails with towering signatures look like the author doesn’t respect their reader’s time.

I’ve seen enterprise customers ask about including detailed company information, legal disclaimers, and promotional banners all in one signature. The answer is usually to pick one: either include the banner or the disclaimer, not both stacked below a full contact block.

Image sizes by email client

Each major email client handles images differently. Here’s exactly what each one does and what you need to know for each.

Gmail

Gmail proxies all images through Google’s caching servers (the googleusercontent.com CDN). This happens automatically and changes your image URLs when the email arrives. The practical implications:

• Images load fast for recipients — Google’s CDN is global and fast.
• Gmail does not enforce a strict file size limit per image, but very large images (over 5MB) may fail to proxy. Keep images under 5MB (easily done with proper optimization — your logo should be 10–20KB, not 5MB).
• Gmail clips emails that exceed 102KB total — not just images, but the entire HTML. If your signature HTML (including base64 images) pushes an email over 102KB, Gmail shows a “[Message clipped]” link. This is why externally hosted images are critical in Gmail.

The Gmail email signature guide covers the full setup process for Gmail-specific signatures.

Outlook (Desktop — 2016, 2019, 2021, 365)

Outlook desktop is the most demanding email client when it comes to image handling. Several specific behaviors to know:

• Images blocked by default— Outlook blocks images from external senders until the user clicks “Show images.” Your logo and headshot will not appear on first view for new contacts. This is why alt text matters: set descriptive alt text on every image.
• DPI scaling— if the user’s monitor is set to 120 DPI or higher (common on high-DPI Windows laptops), Outlook scales up images without proper width/height HTML attributes. More on this in the Outlook DPI section.
• Images as attachments — older versions of Outlook (2010, 2013) sometimes convert inline signature images to attachments if the image is embedded via base64 or if the message format is set to plain text.

The Outlook signature guide and the Outlook 365 signature guide go into full setup instructions.

Apple Mail (macOS and iOS)

Apple Mail is generally the most forgiving of the three. It handles modern CSS better than Outlook, doesn’t block images by default, and supports retina rendering. The main quirk is that Apple Mail on iOS may re-flow your signature layout depending on the iOS version and display scaling. Keep your layout to a single column or a simple two-column table (content + headshot) to avoid surprises.

Logo dimensions

The logo is usually the largest element in a signature and the one that causes the most rendering problems. Here’s exactly how to size it.

One common mistake: using an SVG file for the logo. SVGs are not universally supported in email clients — Outlook desktop doesn’t render them at all, and neither does Gmail. Always export your logo as a PNG for email use.

If your brand uses a square icon/mark alongside a horizontal wordmark, use the horizontal version in signatures — square logos at signature scale (40–60px) are often unreadable. For more on how logos interact with signature layout, the signature with logo guide is the right next read.

Headshot dimensions

A headshot (or profile photo) in a signature adds a human face to what would otherwise be a block of text. Here are the sizing specs:

For circle crops, you can achieve this in CSS with border-radius: 50%— but be aware that Outlook (before the new web-based version) ignores border-radius on images. The NeatStamp editor handles this by providing a pre-cropped circle image rather than relying on CSS, which works in every client.

Headshot vs. no headshot: I’d recommend it if your role is client-facing. For internal communications or technical roles, it’s less important. When in doubt, check what senior people at your company do.

Banner dimensions

A banner is a promotional strip at the bottom of your signature — useful for advertising a webinar, a product launch, a new case study, or a professional certification.

A few banner best practices: always link the banner image to the relevant page. Include the key information as alt text, because Outlook blocks images by default and many recipients will see a blank box first. Keep the design simple — at 80px tall, you don’t have room for long copy. One headline and a clear CTA is enough.

Banners are available in NeatStamp Pro alongside the rest of the signature builder. The free editor handles the standard signature elements without a banner.

File size limits and what exceeds them

There’s no single universal file size limit across email clients — each has its own thresholds and failure modes. Here’s what actually happens when images are too large.

The solution in every case is the same: host your signature images externally (on a CDN or your company website) and reference them with a standard src="https://..." URL. Never embed them as base64 in the HTML. The HTML email signature guide explains exactly how to structure the markup.

Outlook DPI and scaling issues

This is the most technically fiddly issue in email signatures, and it only affects Outlook on Windows. Here’s exactly what happens and how to fix it.

The problem

Windows allows users to set their display scaling (DPI) as a percentage: 100% (96 DPI), 125% (120 DPI), 150% (144 DPI), or 200% (192 DPI). Modern laptops with high-resolution screens often ship at 125% or 150%.

When Outlook renders an email, it uses the GDI rendering engine (inherited from Microsoft Word, which is the actual engine behind Outlook’s HTML renderer). GDI respects Windows DPI. If your image is 200px wide but you only specify the width in CSS (not in the HTML attribute), Outlook scales it up proportionally to the DPI setting. At 150% DPI, your 200px logo becomes 300px.

The fix

Always set both width and height as HTML attributes on the <img> tag, not just as CSS:

<!-- Wrong — only CSS width -->
<img src="logo.png" style="width:200px;" />

<!-- Right — HTML attributes AND CSS -->
<img src="logo.png"
     width="200"
     height="53"
     style="width:200px;height:53px;display:block;" />

The HTML width and height attributes tell Outlook’s Word-based renderer exactly how many pixels to allocate — it respects these over DPI scaling. The CSS width/height handles the other clients. You need both.

NeatStamp generates signatures with correct HTML attributes automatically. If you’re writing your own HTML, this is the detail that’s easiest to miss and most visible when wrong. The Outlook signature guide has a complete checklist for Outlook-proof images.

Quick reference table

Bookmark this. These are the numbers to use for every project.

Frequently asked questions