With UNROM it should probably be a 128KB one, since there is one 128KB ROM on the cartridge. Or it could be 16KB PRGs, since that is the bank size the mapper works with.
Also, I don't think an assembler should necessarily have the task of making a UNIF rom. If a commandline-based UNIF builder was made it would allow programmers to use whichever assembler they are most familiar with, just by passing the cart config info to the program in the batch file that assembles the program. Or maybe use a configuration file.