Help with plan Size
Well, you need to do two things. Cubic inches of engine does not always relate proportionately. You first have to recalculate the size of the existing airplane for the assigned power. Then you have to calculate the proposed power and proportion the relative size of the proposed A/C to that figure. There are formulae for this. Would suggest that this first phase be done with the use of published handbooks and some discussion in the aerodynamics section herein too.
Then, once you determine the new ratio, within AutoCAD is a command function of SCALE. You simply capture the entire drawing and insert this new proportion figure. In moderate sized models, there will be a noticible difference between 0.666 and 0.625 when re-scaled. Takes about three seconds to complete. Then, you need to go in and correct items such as wood thicknesses to more common fractional or metric sizes, which takes seemingly days to find everything.
It ain't simple nor fast, but can be done. It won't fly the same either.
Wm.