Why would you ask for size from serial number if it is not posted? In any event that appears to be about a 57cm frame.

Asking price is below FMV for an RB-1, even without a saddle. Make sure to check that the seat post is free as I'm struck by the sellers comment it is too big and yet 7-8" of post is exposed. It should have Shimano 600 or better grade components.