If you look at the dimensions for Trojan regular size, Trojan Magnum, and Trojan Magnum XL condoms you will find some interesting info. They all have a ring or base width of 2 inches. Note that the width doesn't refer to the width of your penis, but the width of the condom completely flattened out. So a condom width of 2 inches would correspond to a girth of 4 inches not stretched out. Obviously, they have to be smaller than you are to be snug enough not to slip off.
The Magnums and Magnum XL condoms flare out at the closed end, where your glans goes. The regulars are fairly cylindrical throughout. The maximum width of the Magnums is 2.5 inches and that for the Magnum XLs is 2.7 inches. Therefore the XLs may feel better if you have a prominant head. The regular Trojan condoms are around 7 1/2 inches long and the Magnums and XLs a little over 8 inches.
The condom just has to be tight enough not to slip off. The tighter the fit, the more radial sheer stress is placed on the Latex and the more likely the condom is to rupture or leak.
I haven't used condoms for years but for what it is worth I am about 5.3" girth at the glans, 5.6" at the shaft and just a fraction under 6" right at the base. I did use the regulars for years and never had one pop but they felt tight. Then I switched to a larger size like Trojan Magnums and they felt much more comfortable, and I never had one slip off. Either the regulars or the Magnums had plenty of length for me. The Magnums had enough room at the head so I never felt the need to try the Magnum XLs.
If you are going to try a larger size than you are used to I would advise you do two things. First try one on before you are with a partner to see how it feels. Second, after you "get in", stop after a minute and check to make sure it hasn't slipped off.