They do have a more scientific system. There is a brand of condoms called "They Fit," available from condomania.com, which come in many sizes, from tiny to large, and are fitted by measurement. You can download a measurement kit and print it. However, make sure it prints out to the correct size. If there is any doubt, they will mail a measurement kit to you. I'm very happy with these condoms, except that they cost somewhat more than regular condoms. I have been entirely satisfied with the mail order service, and they are shipped in very plain packages, with no mention of condom or sex, even in the return address.

Now, back to the Magnum XL. I used to use those, because they were the best available option before the They Fit brand was introduced. They were tighter than they should be on me, but are probably about right for your girth. The They Fit size U22 I use now are considerably larger in girth than the Magnum XL. Although the Magnum XL is 65 mm wide at the head, they taper to 54 mm wide at the base, which is not too much bigger than standard-size condoms. Therefore, even if the head is roomy, the base should keep a good grip on you, and I would not be too worried about them slipping off. (For me, getting them off was the problem!)

Some stores have the Magnum XL in 3 packs, so you can try them without spending too much. Condomania also sells a sampler pack of assorted large and extra large condoms (regular brands, not including They Fit), which might be helpful.