We are looking to send a GIF in our holiday card, but don't know what size file will be good for a Marketo send that will come out clear. If anyone has any recommendations, they would be greatly appreciated!
The important thing to keep in mind here is your audience and where they'd be accessing your email--there's a real difference between a mostly-desktop send and a more diverse audience. That said, if you're linking to a hosted GIF somewhere the more important thing to think about is how it'll display and how fast it will load. For example, if you look at this Intuit email, this is a great example of what not to do; the animation is too fast, the file size is 1.5MB but isn't adding anything, etc. It's really a case of discretion. I noted in this article that average newsletter image size is now about 500KB, and I'd be tempted to use that as a benchmark for animated GIFs as well.
Remember that even though Outlook recently updated Office365/Outlook 2019 setups to support GIFs that older Outlook versions and other, older clients will not support animated GIF and thus you'll want your animation's first frame to be a decent no-animation fallback.