Hey,
Not to sure what the max size is, I think the gallery will just give you an error if the pics are too big anyway - Try it...
As for re sizing, I find alot of my friends who dont require super fancy software are quite happy with the free photo management software offered by Google. It seems to be getting more and more features all the time, and is quite easy to use. Google "Picasa" and give it a try.
If you're images are somewhere around 1024x768 I'd imagine they'd upload fine here.