600x800 is a regular standard for most people. We get thumbnails on one page. Then we open it up and for the most part, we can see everything we need. If it's bigger than 600x800, we can click again and get a HUGE picture but then we need to scroll around to be able to see it. What's the point of that? Most people's browsers automatically fit pictures to fit the screen or whatever. But seriously, anything bigger is a waste of computer space.

Whatever art program you are using should have a RESIZE option or button. In most cases, the program will automatically the Y or X axis for you. You fill in X, it does Y. Or vice versa. There's no need to actually type in 600 height (X) and 800 Width (Y) unless you have it turned off. (or do I have that backwards? Y is vertical, X is horizontal??? TONEGUNS!?!?!) I resize everything and sometimes, if you resize it just right, you can hide a lot of glitches or bad focusing. *cough cough*

When doing a dio, you tend to make them even smaller! I know looking thru old dios, I have shots that look too big. But that's another topic!