I would recommend the .40 size trainer.

Size does make a difference, but from a .40 siez to a .61 size trainer, there isn't that much difference that can really be noticed, especially at the beginning stage. The .61 size will cost more in the initial investment. There are a lot more .40 size trainers to choose from. And there are more .40-.46 size planes for your second plane.

Despite what some people say about buying a ball bearing engine for your trainer, I do not recommend one. Your trainer engine will probably take some hard knocks and many people like to keep their trainer to put a warm up flight on before flying their new, second airplane or to fly on marginal weather days. A plain bearing engine is cheap, easy to run and it won't over power your plane, which can cause more problems than you think. Get yourself a Thunder Tiger .42GP or an OS .40LA and fly the socks off of it.

Guys who put a ball bearing .46 in a trainer are making it harder for themselves. The more powerful engine will require you to carry extra down trim to keep from climbing or run the engine at very low power. I don't have my students run full power on a plain bearing .40, so you obviously don't need the extra power. Carrying down trim to keep from climbing from flying too fast makes you have to hold extra up elevator on final. If you release the up to make a roll correction at low altitude as many new fliers do, you'll bounce down and chip the prop.