Many times there is a sticker or foil label with the trailer VIN number on it that also specifies the tire size.
Otherwise, you will have to make some educated guesses. My first guess is that the tires were very old and this may have been the primary cause of the failures.
A 1994 Nomad 21 ft (closest specs I could find
HERE is the Brochure) is close to 5000 lbs gross weight and had 175/80-13B tires. Personally I would want as large as I could fit under it if you intend to actually tow this rig.
Charles