Board Thread:Support Requests - Getting Technical/@comment-24.188.200.145-20180326210541/@comment-168424-20180326224613

The Special:Upload page should tell you how big (file size) an image you can upload is... i.e. Maximum file size: 10 MB (a file on your computer)

Background images for Theme Designer have a much smaller file size limit (500Kb?).

Like Andrewds1021 said, given varying compression levels and image format variances, the actual dimensions aren't as meaningful.