I would keep the dimensions of the plane and use the smaller propeller. The smaller propeller will weigh less, the wing loading will be less, the sinking speed will be less. The propeller will be a bit less efficient, more importantly, will turn faster and will run out the motor turns more quickly. If you are flying indoors, that may not matter much, since you don’t have the altitude to take full advantage of the energy available from the rubber. But once the contest is over, you can put the bigger propeller on for sport flying outside.
The most important correction for gas turbine plant performance tests involves compressor inlet air temperature. Although a site’s barometric pressure typically varies by no more than 10% over a year, its temperatures may range from 20F to 100F over the period. Because air temperature has a direct effect on air density, temperature variation changes a unit’s available power output. For a typical heavy-duty frame gas turbine, a 3-degree change in temperature can affect its capacity by 1%. A temperature swing of 30 degrees could raise or lower power output by as much as 10%. The effect can be even more pronounced in aeroderivative engines.