Back in the eighties I scratch built a 29" plane with the appearance of a Baby Ringmaster.  Then I made the mistake of using Monokote covering which made it heavier,
It flew ok on an .049 Cox, but it wasn't no Ringmaster!
My then brother in law scratched a bigger plane with allmost 40 inches wing span and it was HEAVY. The Cox .049 struggled to keep it aloft, like having a glider on the end of the lines.
Think small and light when scratch designing for 1/2 a.
Build them, fly them, find out what works, that is the great fun of modeling.