We have an email file size limit of 150kb, which is very large even for HTML campaigns. Since we do not transmit attachments, file sizes rarely exceed 40-50kb even for very long HTML campaigns. We provide image hosting for your HTML campaign images, which also lowers the email size.
Our system has a safeguard in place so that if you do create a campaign greater than 150kb, it will warn you and prevent you from sending that campaign so that you can edit it. Also, we send campaign previews to addresses you specify before your campaign goes out to your entire list. This gives you and your colleagues the opportunity to make sure the campaign looks just the way you want.