I make 4-1/4 x 5-1/2 mostly. I buy my white cardstock at Sam's. It's called Image Plus, and costs about $8 for a pack of 250 sheets. I use it for my card bases, (cut in half, a sheet makes two) and then attach the cardfronts, so my cards are white inside.
I also use the Sam's cardstock for stamping images and coloring. I know there are folks that buy more expensive cardstock, but I'm happy with it and like it a lot.
Very rarely, usually for a grandchild, I make a larger card, 5 x 7, or a square card, 5 x 5. But then you have to make a special envelope, so I don't do that much. I get my envelopes at Sam's, too, and they are a standard invitation size and fit the 4-1/4 x 5-1/2 size cards. I like to put a liner in the envelope to match my cards, and frequently do that.
Here's a link to a great little tutorial for lining envies to match your cards. It's really easy to do and adds a lot I think.
Ink Stains: Pattern Paper Lined Envies!