G'day
I second John Buckner's comment - the Sig Kadet is a 60 size model which only needs a 40 size engine as it is nice and light. This means it does not need to fly fast to stay in the air.
I have three at the moment. My favourite has a 62 Saito in it but I recently converted another to use a 46 class electric motor and it flies really well too. With a 5000MAh 4 cell battery, it will fly for about 30 minutes at training speeds.
I also like the Kadet LT-40 too. It is quite large for a 40 size trainer.
As has been said, big planes generally fly better than small ones. I have three of the Four Star series - 40, 60 and 120 and the 60 flies better than the 40 and the 120 flies better than the 60.
The Sig Kadet is also available as a kit (which is my favourite version). It does not have ailerons (which I like) but you can easily add them if you want to. It is a great plane to build and to fly.
Cheers
Mike in Oz