The .40 size trainer is the most popular, but there is nothing wrong with the PT-20. I've flown a lot of smaller planes and they fly just fine. They are also less expensive to run - your gallon of fuel lasts a lot longer.

I've got a Balsa USA Stick 30 framed up just for a play around plane - it's in the same class the PT-20, and I've built it up for 3-channel (R/E/T) and will power it with a .15. For the PT-20 I'd with the OS .25 LA - it will be excellent for it, just don't go any larger as it is not required.

And as Missileman says, get into a club so you can have an instructor get you off on the right foot. I'd actually get into a club before you start building as there will be the experience there to help give you some additional pointers with the build also.

Hogflyer