With a homemade converter, the idler motor is small- anything big enough to spin the 3 phase motor used to make the converter. The converter motor needs to be bigger than the 3ph on the machine tool in order to develop enough power, as mentioned in another post.
I made mine with a 5 hp 3 phase, spun by a half horse single phase motor. It runs a 3 hp 3 phase turning lathe and a 3 phase Milwaukee mill. I had the motors and I think both switch boxes, just had to buy some cords and plug-ins. google HM phase converters for wiring diagrams and examples.