If you want it to fly similar to the full size, then you want to retain same the proportions.
For a fast or really "sporty"plane you can get a feel after the reduced size drawing is made for what size engine, tank and radio gear will be a good fit.
The other method is to already have your equipment in hand and then design the plane around it. This is my favorite path.
In the case of your electric power I think you will find a greater variety of choices and plenty of expert [sales] advice once you have arrived at what the size and projected weight will be of your project.
Bear in mind that there is nothing wrong with borrowing the structural methods and details that have already been worked out for whatever "scale" you decide to build.
The methods and materials will vary some what for a given size plane so that it is built strong enough to withstand the forces of flight without taking on extra, fun robbing and performance robbing weight.
Spend as much time as you need studying plans that are out there scattered here and there if you do a little bit of searching. Some of the RCU forums are a jackpot for this stuff.