The 'biggies' with mag-mounts are the strength of the magnet and the amount of surface area that magnet has with the metal it's attached to. That will determine the size of the antenna it will support and the amount of coupling to "ground". I haven't seen a mag-mount yet that was 'too big', but that depends on the use/abuse you put it through.
I think the mag-mount I have is a Larsen, but I just don't remember for sure. It does what it's supposed to do, and I'm happy with it. It is definitely capable of holding a typical 2 meter antenna just fine (a crowbar would be very handy to have when removing it).
Have fun.
- 'Doc
(I can't say a mag-mount is my preferred method of mounting an antenna, but I still use them.)