The max file size for a given image being uploaded is listed on the image upload popup:
As you can see, the max file size is 2MiB (which is just about 2MB total).
Image dimensions, I don't know if there's such a restriction, you'd have to ask Imgur. But file size limit is 2MB. I don't know if Imgur has a dimensions restriction, but 2MB doesn't leave for much in terms of super 'high resolution' files.
As noted in comments, this is only a restriction if using the StackExchange embedded Imgur upload system. If you upload something to your own imgur account, or run it from your own web server, you are not bound to that 2MB image file size restriction.