Indeed, only way to know is to try em on. When I first started having sex it was the sickly orange colored trojenz something or other I used, regular sized condom. I'm 5.5 around at the widest point and a bit less at the base, and even with that I found the condom ring would leave a nasty red ring after a session. magnums (not the XL) are very close to the same size as a regular condom, but that little bit of extra room goes a LONG way. You'll be happy I think.
On a related note, if you are indeed having sex for the very first time, take my advice and buy a bunch of condoms. You could get some regular sized ones from school or family planning for free I'd think. Anyway, the reason for this is practise. Practise putting them on, jack off using them a couple of times, etc etc but get comfortable handling them and your cock at the same time, it'll be one less thing to worry about on the big night!
Hairyguy