For the .61's I fly in pattern type planes I use either an 11 X 7 or 12 X 6. For the US 60, with a .61 2c, I'd lean more towards the 11 X 7 since it'll give performance closer to the older pattern planes (the US 60 is an outgrowth of the Bridi Kaos 60). Not sure on an electric conversion - you may want to ask they guys in the lekky forum since you are getting into more specialized, rather large and very expensive motors/esc's/voltage regulators/batteries.

Hogflyer