Avatar pictures on PSF are ideally 150x150 (pixels). The site will allow you to upload images as small as 90x90. There is a 300k cap on file size. This is because we are hosting these images here on the site, and image directories have the potential to become quite large.
If you do not use an image manipulator on your computer, or are unfamiliar with resizing images, there are online image resizers like this one that are easy to use: http://www.simpleimageresizer.com/
If you're having trouble, feel free to post in the Suggestions thread, PM me, or send me an email.