Oh Suzi - that sooo sucks!
The most common sizes for most message boards are 620x480 but I personally think that is too small most of the time... I like mine a bit bigger... 800x550 works well for me.... Sometimes I do about a 1000x750 too... It just depends on what I am uploading. But for most things that middle one is perfect for me. It's really a personal thing (I think) and you can do whatever your comfortable, because as far as I know, the Dis doesn't have any sizing limits.
If you need help with PHotoshop... I am happy to help when I can - otherwise
Youtube and Google are your friends for tutorials, however, just be careful about the websites you open, when ever I google computer related stuff, it always seems to come back with some sort of virus in one of the pages I open...
D~