How large are the "full scale plans"? Are you using Autocad, or a 3D program? There are full scale plotters for commercial use on the market for converting full scale scale raster drawings into vector lines. But for the home use I can only imagine to physically measure every part on your plans and scale them down by whatever your desired size might be. Without a 3D program it would be a somewhat cumbersome task to down size a full scale airplane, though.