As there is little correlation between engine size and power output, the answer can only be determined by looking at generator manufacturers' sizing charts.

I's start looking at the power needed to drive them. For 2x1.5 hp = 3hp consumption, I'd assume to use a generator at roughly 75% full capacity including losses (80% efficiency), so, a generator input of at least 5 hp as an initial value would be needed. Just choose the engine that best fits this power with the required generator drive speed.

Just my opinion, of course. No experience at all.

I maybe reading between the lines, but I think that you are asking what size motor (in cc) would be required to power an air conditioner compressor normally used in an automobile, is that correct?

guest; rule of thumb 2 HP per 1 KW perry