I have an Acer Aspire E360 with Amd athlon x2 3800+, how big of a processor can

It's hard to tell; not enough specs available on the motherboard chipset. For such an old setup, I'd check craigslist for micro atx combo deals for socket 939. That way if the board doesn't work on your acer board, you'll be able to use it on the other one. If you don't want to be stuck with components that won't work on your acer, or don't feel comfortable changing out a motherboard, I would just leave it alone.