Way to big a file size...I'm not sure the exact size that is currently allowable, but its somewhere around the 3000x3000 resolution and something less than 20MB's in size. I upload PNG files at 3000 resolution and under 10MB's and don't have any problems.
File size ... unlimited. (Nothing is hard-coded into the site on this note.)
And as mentioned above, 10 Mb. ... total file size ... seems to be the magic number before one may encounter being 'timed out' and experiencing an unsuccessful upload.
/\ That is to say, if you have a DSL or cable (broadband) internet connection.
Image resolution size ... restricted to 3000 X 3000 pixels, a side.
Anything larger than the above stipulation, will most likely result in an error message being displayed ... stating the acceptable and aforementioned parameters/considerations.