How many different sizes of T12 slimline (single pin instant start) lamps are out there? I’m familiar with F96T12 (8 foot), F72T12 (6 foot, often found in gas station drink coolers), and a somewhat rarer bird, F48T12 (basically a slimline version of F40T12 rapid start Bipin lamps).

My GE catalogs list 24",36",42",48",60",64"72"96" the oddball lengths (42",60"+64") were more than likely used in display cases or something similar (I bought an old display case that has a F64/T6/WW in it ! I'd never even heard of a T6 !

There's also the T8 versions.

I’m familiar with those. My work had F96T8 slimlines before the fixtures were LEDified.

The only T6's I heard of is the F42T6 and the F64T6. These were single pin lamps commonly used in display cases before the T5 HO lamps were made. Slimlines are mostly referred to the F48T12, F60T12, F72T12, F84T12, and F96T12, and of course the T8 equivalents.