Standard baseball card sized. Or basically anything that will fit Magic: The Gathering will fit.
I have three ring binders for each faction, with these
Ultra Pro Sheets for my cards. For playing, I like to use
Top Loaders. The only problem with the top loaders is that they are a bit bigger then the cards, and sometimes the cards slide around a little bit. Not much, but enough to mess up damage marking tracks. I've seen some people get really thin plastic card sleeves and then put them into the top loaders and that seems to work well.