Email template size: width, weight and what actually clips

The size numbers that actually change outcomes are few. A body width of around 600 pixels still renders predictably everywhere. Gmail clips a message once the HTML exceeds roughly 102 KB, showing a "View entire message" link and hiding everything after the cut. Mobile Gmail shows roughly the first 35 to 40 characters of a subject line. Everything else in the usual size guide is convention, not constraint.

Clipping is the one with a compliance edge. What sits at the bottom of a marketing email is the physical postal address and the unsubscribe link — the two things CAN-SPAM requires. If Gmail clips them away, a recipient who wants out reaches for the spam button instead, and complaints are the metric that ends your sending. Google asks bulk senders to keep reported spam below 0.3%.

Width and layout

There is no useful "ideal height". Length should follow content; a long email that someone scrolls is fine, and a short one is better for cold outreach. Height only becomes a problem via total message weight.

Weight: the 102 KB line

Gmail's clipping threshold applies to the message HTML, not to the images it references. That is good news, because it means the fix is almost always in the markup rather than the pictures.

What inflates HTML past the limit:

• Exported templates from design tools with thousands of lines of nested tables and repeated inline styles.
• Long tracking query strings on every link, multiplied by every link.
• Base64-encoded images embedded in the HTML. Avoid these entirely — they count against the HTML size, and many clients will not render them.
• Commented-out blocks and conditional markup left in from an old version.
• Very long preheader padding tricks.

Practical budget: keep the HTML under 100 KB, aim for under 60 KB, and keep total image weight under roughly 1 MB for a marketing email — well under 200 KB for anything cold. Images are fetched, not embedded, so they affect load time and the first impression on a slow connection rather than clipping.

For cold outreach, the honest answer is that the whole question is irrelevant: a three-to-five-sentence plain-text email with one link is a couple of kilobytes, cannot clip, and outperforms a designed template anyway.

Subject line, preheader and signature

• Subject line: write for the first 35 to 40 characters, because that is what a phone shows. Desktop clients display more, so put the useful words first and let the rest be a bonus. There is no penalty for a longer subject line, only truncation.
• Preheader: the first text in the body becomes the preview snippet, roughly 40 to 100 characters depending on the client. If you do not set one deliberately, the client pulls whatever comes first — which is often "View in browser" or an unsubscribe line. Treat the preheader as the second half of the subject line.
• Signature: three or four short lines of text. No logo image, no social icon row, no banner. A signature image is a second request your recipient's client has to make, it fails to load in blocked-image clients, and on cold mail it adds nothing the text does not already say.

Images, dark mode and the rules that matter

• Never make an email a single image. It is unreadable with images off, invisible to screen readers, and a recognisable pattern. Real text is also the only version a filter can assess charitably.
• Always set width and height on image tags, so the layout does not collapse before the images load.
• Always write alt text. It is what a blocked-image recipient reads.
• Dark mode: provide a logo and any icons that work on both light and dark backgrounds, usually by giving them transparent padding rather than a white box. Avoid pure-black or pure-white backgrounds in your light design, because several clients invert them unpredictably.
• Use PNG for logos and flat graphics, JPEG for photographs. Keep animated GIFs small and readable from their first frame, since Outlook on Windows shows only that frame.

Attachment and message limits

Two things to remember. First, attachments are MIME-encoded, which adds roughly a third to the raw file size, so a 20 MB file is not under a 25 MB limit by as much as you think. Second, the recipient's limit applies, not yours — and on a corporate tenant it may be lower than the default and may block the file type outright.

For cold email: send no attachments. A link to a page is better received, better tracked and far less likely to be filtered or stripped by a gateway.

What not to do

Do not design a cold email. HTML templates, hero images and buttons signal bulk mail from a sender with no history. Plain text with one link is both safer and more effective for one-to-one outreach.

Do not embed images as base64. It pushes you toward the clipping threshold and many clients will not render it.

Do not put the unsubscribe link and postal address only at the very bottom of a long HTML email. If the message clips, they disappear. Keep the HTML small enough that it cannot clip, and add List-Unsubscribe with an HTTPS URL and a mailto, plus List-Unsubscribe-Post: List-Unsubscribe=One-Click per RFC 8058, so the client offers a one-click opt-out in its own interface regardless.

Do not send HTML with no plain-text part. A multipart message whose text alternative is missing, or is just "view this in your browser", is a weak signal.

Do not optimise sizes while your placement is unknown. A pixel-perfect template in the spam folder renders perfectly for nobody.

Where MailPilot fits

Template size affects whether a message renders and whether it clips; it does not decide whether the message arrives. MailPilot covers that side: it warms mailboxes over OAuth on Gmail and Outlook or over SMTP/IMAP for SendGrid, Amazon SES, Mailgun, Postmark and custom SMTP, checks SPF, DKIM and DMARC for each sending domain, measures real inbox placement per provider with a seed-inbox network — including whether Gmail is routing you to Promotions rather than the inbox — and tracks per-mailbox reputation daily.