If I brew a 5 gallon batch and a 15 gallon batch on the same system, would I see a different hourly evaporation rate between the two batches? For the sake of the question assume all other variables (boil gravity, boil strength, etc.) are the same between the two batches.
With everything else being constant, energy in, losses to the environment etc.. all apart from the boil volume, then the evaporation rate (mass of water evaporated over time) will be constant also. Put another way, the evaporation rate is independent of the amount of water already there - with everything else constant bar the volume, the energy going into and coming out of the system is the same, so the evaporation rate must be the same.
(With that in mind, you might think that it's curious that BeerSmith wants evaporation as a percentage. If evaporation really were a constant percentage, then you'd never be able to evaporate all the water, just a part of it (the evaporation percentage.) Yet boiling dry is something that anyone who has left a saucepan on the stove can attest to being very real occurrence!)
The reason is, in practice, changing the boil volume has knock on affects, so it's not practical to keep everything else constnat. For example, with a larger volume of water, there will be a larger surface area in contact with the sides of the kettle, and so more losses through the walls of the kettle. So even though the boil vigor is the same and the amount of heat going in is the same, more of that energy is lost to the environment with a larger boil, reducing the evaporation rate.
This loss can be quite significant, so in practice smaller boils will evaporate at a faster rate because there is less energy lost to the environment for the same boil vigor.