1/5.5 scale is probably not a bad scale to attempt.
The Rusjet MiG-29 is 1/4.75, so you're a bit smaller than that. Those guys have their process down though, as far as maintaining weight of the structure.
The Skymaster ARF is 1/7, so now you can get good predictions for weight targets.
If you want the model for competition, it would be smart to keep it within the IJMC weight restrictions of 20 kg dry weight - that way you don't exempt yourself from that event. And, as you start modeling the aircraft surfaces and start doing internal arrangements, it's relatively easy to edit the scale as needed (up to a certain point!)
