Scale RC Model Airplane Design

1. Decide which airplane to model and the aim of this modelling project
Do you want it to fast, leisurely, sportive/aerobatic?
Do you want simplicity, lightness, smallness, durability? It is easy to have complex, heavy and large design with many structural weakpoints and take a long time to assemble or even change a battery.
2. Decide on wing loading of model
Find the wing loading of similar flying models that has the flying characteristics you can accept.
8 oz/sq ft is fast for a small model and as wing loading goes higher, the more difficult the flying.
3. Guess the flying weight of model
Gather as many of the equipment and weigh them. This means the motor, esc, battery, receiver, servos, horns, wheels.
Guess the weight for additional components that you didn't weigh or have to fabricate, e.g. pushrods, landing gear.
Guess how much your airframe is going to weigh.
Add all up and the total is the target flying weight of the new design.
4. Decide if the model will have sufficient thrust
Obviously, more thrust is better than marginal thrust.
you got nothing to fear if the thrust to weight ratio is 1:1.
5. Work out the wing area required for the model
Since you have the target flying weight, you can find the target wing area and in turn the target wingspan and such parameters.
6. Decide if you can build the airframe to the target weight
If it will be heavier, either resize it upwards, consider changes to the equipment if necessary, or use alternate construction design.
7. Visualisation of model
Visualise the construction method of airframe, mounting of equipment and components and make changes to the construction design if necessary. Crashes are inevitable, but some durability is always good.
8. Draw out the design if that helps or just start building

Generally, keep to a specific range of size, this way, equipments can be used interchangeably.