I used Trojan Magnums for a long while, never bothering to try the XL, but I tried the XL recently and was happier with it. I have not tried the Trojan Large or the Durex XXL because I've read that they're for long cocks that aren't very thick. I'm 8" long and 6" around, although I'm wider from left to right than front to back and that has proven to make a difference when it comes to condoms. So girth of the condom is what's important to me, which is why I haven't bothered with the Large or Durex XXL. I have some observations about the difference in the way the two fit.
I tried the Magnum XL at the suggestion of a girl I was having sex with at the time. The regular Magnum has a very even width across the entire thing and it felt uniformly snug and no part of it moved at all during sex. I was personally okay with that because it made me feel secure in the knowledge it wasn't going to fall off and create an unsafe situation (that's a real mood killer). What she told me is that women are used to the feeling of the skin on the penis shifting back and forth during sex, so the Magnum's lack of movement made it feel to her like I had a condom on. The Magnum XL has more of a flared shape to it, it's tighter at the base of my cock and definitely leaves a mark after I pull it off.
At first I was a little disturbed by the movement of the condom on my cock and pulled out momentarily only to find that the base stayed secure and it was just the rest of it that stretched and slid, the condom had not broken either. That was what sold me on the XL.
The length felt the same, perhaps it could have been a little bit longer but it wasn't visibly noticeable, if anything the slightly extra length provided that nice slipping action. That same girl told me she liked the XL much more, it didn't feel to her as if I had a condom on because of the small amount of slip. I was a little bit bothered by how tight it is at the base but since it feels better for the women I've had sex with since the switch I'm okay with it. I mean if it's tighter at the base then that's just like having a built-in cock ring.
I'd say if I were 6.5" in girth and beyond at the base I might have had a bigger problem with the XL, but that amount of girth is pretty much a universal problem for condoms anyways.
After making a switch I can't claim to want my cock to be any bigger, it's not so long that it pokes the cervix and if it were any thicker then condoms I think would be a problem. Ultimately, I would be unhappy if my safe sex choices were limited due to the size of my cock.
I've had a brush with the possibility of having contracted an STD, thankfully everything turned out negative, so it gave me a dose of reality I needed to face. If you're a single guy like me that hopes to one day have that girl of your dreams that you spend the rest of your life with and breed some more humans, an occurrance like that can be really scary. So yeah, even though I'm an LPSG member and I've seen cocks way bigger than mine on this board I'm not the least bit jealous because I have more safe sex options.
My advice is, if the regular Magnum is snug enough not to slip at all, try the XL. It'll be slightly tighter at the base and that slight slippage of the rest of it will feel weird at first but actually make sex more enjoyable. If you need to be really sure, try one during masturbation then if all goes well try it with your partner and I'm pretty certain they'll enjoy it more.