What would be the electric motor size equivalent to a Cox .020 Pee Wee engine?

I think a 50-60 watt brushless would be about the same. I could just barely get a Goldberg Gentle Lady off the ground with one years ago.

The standard rule of thumb would say 40W... but you'll probably be using a far larger diameter prop than the PeeWee could run, so you'll have a lot more "effective" power..... unless you need a speed demon... and then you'd went a high Kv 40W motor with a PeeWee-sized prop on it!