Don't double post -- you know better. Soon enough when someone knows the size they will reply. Dont worry your topic wont disappear.
I personally use 500 width, 850 Height 500 width is roughly where the little box with the guild name and description end. and height pretty much matters on your style of layout. If you want to, when you're done it, show me.
I usually use 500 by 700, but it is mostly down to personal preference. Just make a really basic one, put it up and have a look to see what you think. You will then get an idea of what size your real one should be