The reason you see a variety of size recommendations for rotary converters is due to how starting the load is. In our case, the mill motor starts with very little load. The bridgeport motor is a super heavy duty motor and sometimes you see the 2hp also rated for 3hp for 30 minutes. It can also put out much more HP for shorter times. Because of the heavy duty nature of the bridgeport motor, I would go with a 5hp converter. I made my own, as many have, it is very easy and pretty cheap. Another benefit of a 5hp is that you could run a second similar sized machine at the same time should the need arise.
I am sure you noticed how bridgeport 2hp motors are as large, or larger, than 5hp motors you see out there.