I have a 2000 4dr Cavalier and might be moving a very muddy and bumpy road. I was wondering what the biggest tires that will fit on 15" rims are? I'm hoping somewhere around 215/75 so I could use some good A/T tires. I'd end up getting some cheap 15" steels too.

215 75's would definitely rub, the biggest I think that would fit are 205/65/15's or maybe 215/65/15's. Falken sells a 205/65/15 rally tire that you might like.