They're all around the same size at the base, so nobody really needs to worry about slippage. You can post your dimensions, but penises aren't just perfect cylinders, so you can only get an approximation. If you're thinner at the top but really thick at the base, then a magnum could be too wide. Shape effects how it rolls on, and circumcision really changes it. The best advice is to just shell out a few bucks, try a bunch of larger sized condoms on (just masturbate lightly) and see which one feels the best.
I think whoever said 5 3/4" girth is the _minimum_ for a magnum is just reading that off of a Yahoo! answer (I just looked up someone saying the same thing), but really I'd say just above 5" around would be the minimum. For condom sizes, the 'width' isn't meant to compare to the width of your dick - is the width flattened out, so 2 x width = circumference. A magnum is 2.5" at the maximum around the head, and tapers off quickly towards the base, so as long as you're a little above 5" around it should fit alright and not be nearly as tight as a regular condom.
So a standard condom's width is 2": that's 4" around. If a regular condom is supposed to fit most people, it would make sense to assume someone a little below the average would be fit just fine, so 4.5 or 4.25 would wear standard without trouble. If a magnum's 5" around (at the max, and really closer to 4.5" around for most of it), then 5.25 would be about where you'd expect it to fit just fine, and 5.5 would be where you really would be much more comfortable in a larger condom.
I'm about 7 1/4" long and 5 3/4" around, and switched to magnums after I trying a big sampler pack and finding that the trojan intense ribbed felt way better than anything else (never bothered to measure anything -- tried it blind first so I would actually pick the best one), except that I really hated those ribs along it that just flattened against my dick and felt uncomfortable. I looked it up and saw that the magnum was exactly the same size, just without ribs, so I tried it and it was great.
I tried a Magnum XL too. It fits nicely along most of the way, but the top fifth of my dick head is a little loose. This is why you really need to just try them on, since that fit problem is more about the shape of my dick than anything else, since dick heads vary wildly in size and shape, and depending on how it tapers off towards the tip it will change the overall feel of the condom a lot.
But everybody here is right: the lubricant stinks and is tough to wash off. It is, however, really effective lube and does a good job. Most girls complain about the smell too.
My favourite protection is monogamy with a girl on BC, but that's not for everyone. Nothing beats skin on skin.