To estimate the rolling diameter, add twice the width to the bead (ETRTO) diameter. Example; The 28mm x 406 Conti GP should be 28+28+406=462mm or 18.2 inches, which is pretty close. This method will get you fairly close, but manufacturer size ratings are sometimes inaccurate.
As you can see, the 406-20" is closer in size to the 369-17" Moulton than it is to the 451-20" Conversly, the 349 16" is much closer to the 17" than it is to the 305 16".
305-16" and 406-20" are sometimes refered to as the "American" or BMX sizes and are traditionally listed as having widths measured in decimals of an inch (ie. 1.75). Both closely approximate their nominal sizes of 16 & 20 inches when fitted with 2" "Balloon" tires.
349-16" and 451-20" are refered to as the "British" or "lightweight" sizes and are traditionally listed as having widths measured in fractions of an inch (ie. 1 3/8"). 451 closely approximates its nominal size of 20 inches when fitted with 1 1/8" tires.
369 is the Moulton 17" used on AM series (Space Frame) Moultons. I've read that the size was chosen to allow the same rolling diameter as "18 inch" sew-ups, for weekend racers. However it would seem more logical to match the brake track diameter. That Dr. Moulton would introduce an entirely new size, so close to the fairly common (in England) 349 he had been using, strikes me as more than a bit odd.
Tire selection clearly favors the 406 and 305 sizes, especially for North America, where the other sizes are specialty items, hard to find outside of large cities. However, the 349 Primo Comet is nice enough to make me seriously consider that size.
I don't have any info on 18" 400mm, or 450mm tires, though I know that some recumbents are built with them. If somebody wants to send me sources for these sizes I'll be glad to add them to this listing.
There are at least 3 other "20 inch" sizes which have been used around the world, but they are irrelivant to the purpose of this list. M.S.