schwinn bb size
 

i have been digging on the web to no avail, so i throw myself at you, the keepers of all bike-knowledge.
what kind of bottom bracket (type and size) is on a 1988(?) Schwinn prelude
thank you
thank you
thank you

Measure the bottom bracket shell width. Also, what model and type of crank does the bike have? Almost certainly the threading is English.

the crank is sakae sx (double)
unfortunately, i have nothing with which to measure the bb shell
are there any webpages with specs for this bike? (i couldn't find anything)

The shell is highly likely to be 68 mm wide and English threaded. Your crank has a JIS square taper and I expect the original bottom bracket is an old cup and cone, loose bearing type, probably an SR like your crank. If you can disassemble it, you can measure the axle length.

As to replacement, nearly any Shimano square taper cartridge bottom bracket should work. Say a UN 53 or UN73 and I'd try a 115 mm length as a first guess.

I agree with HillRider that its likely either a 115 or perhaps a 118. The UN53 or UN73 are both excelent replacements for the money. I've found them on sale for less than $20.