Magnums aren't for length, Mags are for girth
pretty much all condoms you roll out go up to like 7 inches or something. but magnums are 15% bigger in terms of girth than regular condoms. Hell, there's even MagXL which is 25% if i recall correctly.
i've tried all three and found regular too tight and it would chafe (not fun), mags were comfy and tight enough to not fall off (though i don't use the trojan ones, i've had bad experiences with trojan, Durex all the way!) and the MagXL fit, but was too loose and would pop off in the middle of thrusting.
as far as lasting longer, you could get one of those condoms with the numbing agent inside. or if it's the opposite you want, get teh thinner ones (usually called thinsations or something)