Does it mean that the slots only accept up to a 32k chip, or the entire board only allows 32k (Say a 16k prg and 16k chr)
256 = 256 kilobits = 32 kibibytes.
I didn't know they used these numbers with CNROM boards too, but in NROM boards the number is exclusive to the PRG-ROM chip (128 = 16KB, 256 = 32KB), and there are always 8KB of CHR-ROM.
I imagine that in CNROM boards the number is the maximum size of the PRG-ROM chip too. I don't know if they ever limited the size of the CHR-ROM chip to less than 32KB...
According to NesCartDB, CNROM-128 never existed, so I'd assume they always came with 32KB of PRG-ROM.
On a few boards I've seen there is solder pads that would allow for a 128kbit (16 kB) CNROM RPG. Maybe the -256 boards doesn't have those pads tough.
How big is Star Force (U) again?