In short, the 5" reflector is for the flash head that fits Nr 5/Nr 25 bulbs (bayonet base) and the 7" reflector is for the flash head that fits the Nr 11 (and larger) bulbs with screw base.
Guide numbers are printed on the flash bulb packaging... or readily availalbe on the internet. Antoher source is the Kodak Photoguides of the 1950's 60's and early 70's. The Kodak guides have flash calculator to help with GN vs distance as well as the effects of shutter speed.
Bulbs give a big flash and that is their benefit. You can do the same with a strobe, of course... but those small bulbs really pack a lot of light in them.