The first answer that comes to mind is “mail should be as small as possible”. But for a moment, let's pause to ask ourselves: is there really a set limit?
Thanks to Michelle Klann, researcher at Email on Acid, we have a precise answer: 102kb.
In a recent post on email size, Michelle explains:
“If your email exceeds 102kb, Gmail will display the first 102K and then cut off the rest with a few different variations depending on the display device.”
We list some of the consequences of exceeding this limit in Gmail, and the worst case:
After reaching 102k, the mobile version of Gmail for the iPad seems to not offer the links to view the full message.
Fortunately, images do not count towards this total. The attachments are not added to the referred weight, only the content in html, or text.
The bottom line is that if the size of the HTML file is close to 102kb, the email is too long. So let's think about how to improve email messages, including the alternative of placing complementary content on a landing page for easy reading in the browser.