parabellum

are you asking what percentage you should use when enlarging?

that depends on what plan you have in mind.

lets say you have 40 size plan, and want to make it 60.

40 and 60 are size in volume so theoretically, you want to increase linear dimensions
in cubic root of the volumetric increse in size.

40 to 60 is 50% increase in volume, but only 15% increase in length.

formula is

muliplication factor = cubic root of ( size of engine you want to use / size of engine the plan calls for)


but, it is probably easier to use wing span of other 60 sized airplane to calculate
your multiplication factor.

mf = wing span of 60 sized plane / wing span of plan you have


if you want percentage

mf x 100

percentage of increase

(mf x100) - 100