I decided to go for two sizes. Those sizes are AA and C2032. The AA is used in my GPS, the lights for my bicycle, my flashlight and my Psion. The C2032 is used for the Photon lights and as a backup battery for the Psion. New items are required to comply with the standard.
As required items (e. g. the light for the bicycle) are only available with AA batteries the range for my choice was pretty narrow anyway.