The battery size is not based on the running load directly. The size is based on starting requirements and reserve requirement.
For instance, a CB750 requires about 200 CCA (Cold Cranking Amps). My CB360 requires 110 CCA. So if you have electric start, the CCA rating is the dominating requirement. A 110 CCA battery is usually about 12 Amp Hours.
If you don't have electric start, then the requirement is usually enough reserve AH to keep the bike running for a period of time without charging. For a cb360, standard lighting and coils, this is about 9 AH, or 25% less then the electric start version.
The battery also serves as a reference for the charging volts and as a place to dump excess charging current (On permanent magnet alternator systems). The alternator supplies all the electric needed for running the bike. However, at low RPM's the alternator usually does not put out enough current, so the battery supplies this while idling (at red lights for instance).
Too small a battery means the battery will discharge quickly at a light or long idle time. Low voltage is prone to make the engine misfire or stall.
While many have said they have installed very small batteries, I doubt the batteries last very long, and they probably would have problems (they may not admit it) if the bike idles much time.
Depending on your riding habits, the original size always works best. If you have reduced the lighting load, then the charging system will actually try to overcharge the battery. A smaller battery will suffer with the increased charging (Less load for lights means excess charging load). Sonreirs regulator will help that a bit, but since you stated you are keeping the electric start, the battery you use should have the same CCA as the original. For your bike, I would assume the 110/120 CCA rating is close to stock.