When submitting cover art to MusicBrainz Cover Art Archive it would be nice to know what the maximum file size, maximum resolution, and required file type are.
This would perhaps inspire users to submit higher quality images also, because the will not have to make a “safe” image size of say, 150x150px.
Right now, when I try to upload a photo it just loads forever (over 16 minutes)…
I don’t think it even gets submitted at all.
Are there any criteria for file dimensions, size, and type?
As far as I know there is no limit.
from How to Add Cover Art, supported file types are gif, jpg, jpeg, png, htm, html, jpe, jfif and pdf. there’s no maximum filesize, but 15MB is “somewhat overkill” (according to the above)
I’d say minimum would be the smallest size given on the cover art page, or 1200px, but small cover art is better than no cover art
I typically upload 1200x1200 PNGs which run about 3-4 MB. Never had an issue with that size.
I go as large as possible. Images can always be auto-downsized if space becomes an issue, never upsized. I believe the Internet Archive had also stated that space is of no consequence 
p.s. try uploading again later, the IA often has short-lived server issues
Also from the page that @UltimateRiff posted: "if you're planning to scan your releases, we recommend doing it at 600dpi.
Note that the page isn’t a concrete guideline. I upload images bigger than 15MB on the regular 
I’ve put in an edit on the wiki page.
Just realized that the progress bar only starts uploading once you click on “Enter edit”. 
The thumbnails didn’t appear so I got confused as to what was happening. Apparently, nothing happened, because the upload only started once you click on the button.
A little off-topic but since we are already talking about it:
As for myself, I try to submit the largest available scans I have and I really encourage others to do so too. You wouldn’t scan the Mona Lisa at 300dpi 
What is more important is actually removing dust and make sure you do not remove parts of the picture while cropping. I see so many scans that are horrible. Also, they often lack information about what the scan really is. Like a picture tells me nothing about if it is the front cover, liner, booklet or backside.