RE: What servo size to use ???
No, I am not familiar with that, but as you said, it's a trainer, so it cannot be that complex nor heavy. If you've never built before, maybe you ought to consult with someone in your local hobby shop or club for a little assistance. It does come with directions, right? I hope so, or you will have fun trying to put that together, making sure all the parts are there and where they go, and in what order, for instance.
But stick with it and you should do fine. Get some help learning to fly it, too. You don't want to spend all that time building and finding and getting advice and help, only to smash it up on the first flight, right?
There are any number of great, and inexpensive ARF trainers, if you get bogged down with the kit process. That may get you in the air sooner as you spend the extra time on the build.