I cant get my battery nut off!!!! ahhhh!!!?

ok i'm out working on my truck (99 dodge dakota w/ an optima yellow top), to upgrade my amplifier wire to a 1/0 kit...the only problem is that i cant get the positive battery lug nut off!!! no wrenches i have fit it they're all too small or too big... wat is the exact size i need to get to fit it?