Size, weight, and type of airframe determine power requirements.

Here is a listing of trainer plane kits from one retailer.
http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=AAB
There are lots of retailers and other choices but the Tower web site is one of the easier ones to use.
The recommended engine size and wing span is listed in the item name.
Example "Goldberg Eagle 2 Trainer Kit .29-.45,63"
Maximim recommended engine is .45 cubic inch
Wing span 63 inches
If you click the links and read the notes you will see the estimated weight of the finished model.
Most trainer type models that recommend a .45 size engine will be similar size and weight.
When you buy an engine, the manual should list a range of prop sizes that are suitable for the engine. You can ask local modelers, or ask on RCU, for specific prop recommendations after you have a plane and engine.