Bigger is always better but there's a point of personal diminshing return. A lot of the stuff I do Pola-wise I scan around 70MB, or ~5200x4500 (scanning at original size with 2400 dpi).
At 200 dpi which a lot of the larger printers default to, I'd end up with roughly a 20"x30". I can't imagine ever going that big but I could if I wanted to. Ideally keep a master file you can size down to whatever print you're making at the time. Always size down, never up if you can help it.
Generally speaking, if you're just wanting an 8x10, scanning the original to size at 600 dpi is a good base (if you don't plan on going bigger than that.)