carolinacurious: As others have said you shouldn't have any problems with Magnums slipping off and they should definitely be an improvement over what you've been used to.
Also I would suggest 2 others: I'm not sure if it's still around but I used to use one called the "Inspiral", the idea is that there's a "pouch" at the head for you to slide around in, feels great. If they're not still around there were plenty of imitators.
The other is the non-latex condom made by trojan (NOT THE AVANTI !!!!!
, you should read up on it concerning effectiveness, I only used them a few times but I really(!
liked them, totally different concept than the inspiral, it fit like a second skin, very good sensitivity, damn near invisible too. It's called the Supra and you can't get it without spermicide. I never had any problems with them, I thought they were awesome but I don't think I'd recommend them for the really huge guys here.
Just did a little googling: they're both still available. Back in the day, I had to special order them but one inspiral site said something about them being in Wal-Mart.
Give them a whirl. Oh, and I'm roughly your size as well. So, if you can't tell, I like the Magnum but like the others a LOT better! Oh, a lot of guys don't know this but a little lube INSIDE the condom can really help the sensation.
Your Mileage May Vary.