Here is my $.02.

Get a good .61 to .75. I had a Super Tiger G-61 and loved it. Once broke in and tuned it was my bast running engine by far.

To say that you want to go smaller would not religate you to a smaller engine. If you scale down the physcial size you may get into something like warbirds or 3-D. Higher wing loading means you need a larger engine for the relitive size. An example of this would be a 60 size cub. You are looking at a plane with 80 some inch wing span compared to a 60 size war bird which would be around 60" so my point here is that engine size is relivant to the specific type of plane not the physcial size compared to your trainer.

I would never put a smaller engine on a plane than the smallest they reccomend.

Other factors you have to look at, which may dictate going larger than a .60 or changing to a 4 stroke..... What altitude are you flying at??????? The higher altitude your field is means you loose air density and would require more power to get the same performance which would mean more displacement and or larger pitch/diameter props. I know when I learned to fly full scale this was very important but flying r/c I never thought about it as I flew at sea level. I now live at 4,000 ft and had to put this knowledge into my flying here. This is very critical especially in the summer where temps are hot and humidity is higher. The more humidity and altitude reduces efficency of your engine power and prop performance. If you search information for DENSITY ALTITUDE and is just as important for our r/c stuff as for full scale. Some times here we have to fly early in the morning to get the best performance in the summer.

If you look through these forums on engine tuning you get every thing from this or that engine is crap and 1,000 different ways to tune. I have been flying for a long time and used many engines. There is not OUT OF THE B OX best engine that you can bolt on and fly.... Every engine requires proper tuning and that needs some learning. No matter what, it has been proven that engines need to be properly broken in and tuned before put in the air. Yes there are people who have and say to just fly it andf get away with it but there are guys like Clarence Lee, world renowned, who stress proper break in and tuning. If you don't know who he is pick up Model Airplane News and read Engine Clinic. This was my favorite in RCM until they went out of publication. Clarence has written many books and designed many engines through the evoloution or free flight and r/c.

Anyway follow these steps.
1)Break in the engine and use a good quality fuel
2)KISS.... Keep It Simple Sir. You don't need a roll away box and 15 different glow plugs or peops to have a good plane and engine especially ina trainer. If you use the reccomended prop by the engine manufacture then you only need a few different prop combintions to fine tune your plane. Yes keep a few spare glow plugs because they do go bad. If you have a sudden "CARB" problem I would change the plug first. Usually, unless you change something major like prop size or nitro content, you don't need to miss with the carb after you get it set. A new engine will not run as good as a well broke in engine.
3)BALANCE YOUR PLANE PROPERLY.....Different styles of planes balance different depending on the performance and flying style you want. If you always start at the reccomneded point then you have a good flying plane, especially a trainer, there is not much adjusting a trainer like a 3-D plane. When I say to balance properly I mean both on the pitch axis like most people do but also on the roll axis (called Latteral Balance) which is over looked by a lot of builders. A lot of crashes are blamed on a lot of things that end up being a problem with balance and then not being able to properly control the plane. This is the simplest and usually overlooked thing you can do to your plane to make it fly right. A Yak or Sukoi and even the Edge 540 are very docile fgor a performance plane when balanced neutral.

So I got a little off topic and drug this out but I want to see people succed. I ahve seen too many people sink a bunch of money and time to throw it in the trash can and give up because of these simple things.