Check out www.comicolors.com.This is a pro comic book colourists site with links to all sorts of comic book resourses pages which should answer all your questions. As a quick guide,comics are drawn on a 11" x17" art board and scanned in normaly at 300dpi.They are normaly coloured in photoshop & lettered in illustrator at this dpi (there are tricks to reduce to work faster,but thats the end result).This is for printed comic book work.If you hope to print at sometime you should produce at that standard & reduce it down to go on the web.When it comes to displaying on the web you can go as low as 72dpi ,and its pixel dimentions not print dimentions that count.