The tank is not sized according to the number of bedrooms or bathrooms. It is sized according to the pump GPM. A 20 gallon tank would be good with a 5 GPM pump. A CSV would let you go with a higher GPM pump.
I'm not sure what their reasoning is to only use a CSV with an irrigation system. An irrigation system draw should be matched to the output GPM of the pump so as to run continuously and not cycle. A CSV really shines when the rate of consumption is less than the pump output.
Horsepower is decided by factors such as depth of well and GPM. Your choice of pump should factor whether or not you go with a CSV. Some pumps float their impellers with individual bearings and a CSV may cause the pump to draw a little more amps than one with different bearings.
I would put the tank in the crawlspace rather than a wellhouse.