Large and extra large condoms aren't actually much bigger than normal condoms in general. The Trojan Magnum is only about a half an inch longer than the normal trojans and it's about 0.175" wider. The Magnum is 1.5" wide and the Magnum XL is 1.75" wide. Normal condoms tend to be about 1.325" wide. Most large condoms are only 1.5" wide.
If the XL doesn't fit you, you're probably out of luck unless you can track down a specialty condom that caters to very think cocks. The best bet is to measure your cock when it's fully erect; you claim to know your length pretty exactly so I'm kind of surprised you don't know your girth as well
