It's just an arbitrary numbers, we chose it a couple of years ago when 256k was the usual size of system flash roms, so the resulting openbios would always fit in a such a small chip with LinuxBIOS and a compressed OpenBIOS payload.
Well, Flash ROMs certainly have expanded since then! I understand that sizes of 8 or 16 Meg are commonplace now, and that 4 Meg sizes are now considered obsolete (or at least, obsolescent) and that 8 Meg-gers are heading rapidly in that direction!
Not on commodity x86 PCs yet :-(
Segher