Trojan condoms are all close to the same size at the base. If you're too small for it, it could feel a bit baggy, but it should still be plenty tight at the bottom. A condom should only slip if the base isn't tight enough.
Their width at the head is 2.7", the shaft is 2.12", and the base is 2".
The 'intense ribbed' (the yellow ones in the variety pack) are 2.5" head, 2.25" shaft, 2" base. This is exactly the same dimensions that the standard magnum has. What's interesting is the standard magnum is actually girthier in the middle, and less so around the head than the magnum XL.
Personally I prefer the standard magnum just because I'm more shaft-wide than mushroom-headed. I'm also uncut so unless it's really tight at the head my forskin slides up and down with the condom.. not sure if it's supposed to do that or not. My solution to condom-confusion is just to be monogamous, tested, and go without a condom.
I do occasionally have to put on a condom (depending on the complexities of female birth control). I liked those 'intense ribbed' when I tried them, but hated the ribs, so after finding magnums were the same size I used those. I really hate how they make you look like "that guy" when you pull one out, so I keep them hidden and unwrap them in secret.
Condom fit is way more about shape than absolute size. If you're having trouble with some, it's really worth your time to just get a fleshlight and a variety box and try dry runs with each. You're far better off if they snap/go loose/feel terrible/smell awful when you're by yourself than having a good night ruined.