How to calculate size of electrical service?
It's 100 amp service. 

Confusing the issue does not help matters. (Virtually) all US household service is supplied at 240V, so whether the service is 30, 60, 100, 125, 150, 200 or 320, it's that amperage at 240 Volts. It is NOT, in fact 120V at 200 amps, because you cannot run a 200 amp 120V load off a 240V 100 amp feed - you can run a pair of 120V 100 Amp loads, but not a 200 amp 120V load - the "pair" are opposite phase and cannot be combined like that.