Here is a list of common yoyo bearing sizes.
These are the most common sizes you may run into. The listed measurements are, in order, Inner Diameter x Outer Diameter x Width. The letter designation is not known outside the yoyo world as far as I know.
Size A - The A bearing fits most Duncan, Playmaxx, YoYoFactory, ZanNavi and many other yo-yos. Measures 5mm x 10mm x 4mm.
Size B - This bearing fits small bearing YoYoJam yo-yos, including SpinFaktors (but not the HG), Kamaitachi, Sunset Trajectory and Matrixx. It also fits the SuperYo Renegade, and the Spintastics Revolver and Eclipse. Measures .250 x .375 x .125 (1/4" x 3/8" x 1/8").
Size C - Fits the large bearing YoYoJam yo-yos including the Hitman, Lyn Fury, Spinmaster 2 and 3, Big Ben, Patriot, Nightmoves 2 and 3, Shockwave 3, the Speeder and the Speedmaker, as well as the YoYoFactory G5 and 888 and many other yoyos today. This size is the main bearing size for competition style yoyos. Measures .250 x .500 x .187 (1/4" x 1/2" x 3/16").
Size D - Fits B.I.S.T., HSpin and SPYY yo-yos. Measures 5mm x 11mm x 5mm. (0.196" x 0.433" x 0.196")
Size E - Fits the Spintastics Tornado, Tigershark, Riot, Great White, Comet, Torch, and Firefly, and Yomega ball bearing yo-yos. Measures .156 x .312 x .125 (5/32" x 5/16" x 1/8").
These bearings are usually made for a specific yo-yo or specific manufacturer.
Size G - Fits all Custom yo-yos. Measures .187 x .375 x .125 (3/16" x 3/8" x 1/8").
Size I - Fits all Tom Kuhn ball bearing yo-yos. Measures .125 x .312 x .140
This has been added to Useful modification and maintenance guides - Clean, repair, tune, fix yoyos
Ref: yoyowiki (http://www.yoyowiki.org/wiki/Ball_Bearings)
ILYY - bearing size is given in the product descriptions on their website:
6 x 13 x 5 - all dimensions in Millimeter [mm]
(Courtesy of Gogol - thanks)