There are only two guidelines that are important:
1) Keep all images under 2048x2048
2) Use power-of-two size textures (32x32, 64x64, 128x128 etc.) for Tiled Background objects only - for sprites, the image size really does not matter.
Since the latest beta, the editor will warn you with a balloon tip if you ever break either of these guidelines, but it won't enforce it (you can ignore the warning if you wish, but it is strongly recommended to stay within these limits).