problem removing + battery terminal

This really isn't cavalier related but that's the car it's on so it might as well go here. Just bought a used 96 Cavalier now it won't start.
I want to replace the battery and I've done this lots before but I think this must be my first side mount.
I unscrewed the - and + bolts from the side and the negative cable came off no problem.
I've removed the cables attached except the + terminal but I can't get it off the side of the battery.
So here are my options as I see them:
- it's very cold here so I don't know if it is just frozen on (seems unlikley)
- the post was corroded so maybe it's sort of stuck now
- there is a larger post? that the smaller bolt screwed into that maybe needs to be removed to get it off (hard to explain but it seems like about an 18mm sized post that extends through the red connector). I tried to turn with a crescent wrench since I don't have an 18mm wrench or socket) but I don't get enough torque on it (slips).
Any tips? I have a picture if I can post it somehow.
I guess I can always cut the cable and try and put a new connector on but that seems a little extreme.

As the battery uses 3/8 inch coarse thread bolts with an 8mm or 5/16 head. They make a terminal where you can use Ford type terminals. This screws in and probablly uses a 3/4 inch socket or open or box wrench. If worn it probably is 18mm. I would try a six point 3/4 inch socket if to big try 11/16 inch. You could pour warm not boiling water on it if frozen. Parts stores carry new OEM type bolts. Use vise grips or channel locks in an emergency don't have the correct tool. These batteries are bad about leaking at the positive terminal.

Thanks for the feedback. I think I can take it from here now.

As the battery uses 3/8 inch coarse thread bolts with an 8mm or 5/16 head. They make a terminal where you can use Ford type terminals. This screws in and probablly uses a 3/4 inch socket or open or box wrench. If worn it probably is 18mm. I would try a six point 3/4 inch socket if to big try 11/16 inch. You could pour warm not boiling water on it if frozen. Parts stores carry new OEM type bolts. Use vise grips or channel locks in an emergency don't have the correct tool. These batteries are bad about leaking at the positive terminal.
11/16th did the trick. I think it is was an extra post added for the command start and stereo.
Thanks a bunch!