I doubt there is an "average" anyone other than site administrators would be able to guess.
However, unless I'm going to ask for a critique of a photo, I try to limit the dimensions to around 6" x 9" at 72 ppi. Otherwise I upload the full size image for critiques.
I only upload 500x375 72dpi watermarked images to help deter in image theft. The file size itself can vary depending what the image is of and if it's in color, mono, quad and so on.
oy..... yeah, i'm having an issue with uploaded size, too.... i changed the size on one of my images 3 times and it still is uploading big. eek. sorry.
Some folks are posting full res photos in the groups. I don't ever look at those because they take so long to load (I'm on a high speed internet connection), and sometimes they keep me from viewing other folks' photos. 600 to 800 pixels on the long dimension should be more than adequate for web viewing.
PictureSocial has some problems resizing images. Therefore, I recommend 800 pixels max horizontal -- it doesnt seem to matter about the vertical dimension but file sizes over 250k do slow things down. The comments recommending watermarking are right on -- it is a good if not necessary idea. Lastly in nearly every group it is better to "post" your images using the camera icon above the window (rather than the "Upload Files" below the window) so your image appears right inside the discussion box.
"Go Cowboys !!! I grew up near Dallas and was friends with many Cowboy players in the early 80's (Tony Dorsett, Too Tall Jones, Drew Pearson, etc..) I also like the expression of the guitarist on the left..."