some of the guide lines you can look at kits or other plans of the same deminsions and see what engines are recommended.
I use these as a guide
weight abot 1 pound .049 area 100-200 sq in
2 pounds .10 area 200-300 sq in
3 pounds .15 250-350 sq in
4 pounds .20-.25 350-425 sq in
5 pound .35-.40 .425-550 sq in
6 pounds .50-.61 550-650 sq in
of course these are sport/aerobatic non 3d guides.scale would be higher weights with the same engine,what size are you thinking of?