The Submission Guidelines in the Help pages suggest that anything over 720 pixels on the longest size will be resized, but in my experience, anything over this size just gets rejected and won't upload.
For resizing images I find photobucket.com the best, there are other image hosting sites...but photobucket is quite simple and it is free..it even has preset sizes in it's edit system, "large" is usually best suited to most sites' requirements