If you have the plans, is there a sail plan, from which you can glean some ideas. For example, if the bottom part of the mainsail is shown, you could extend the lines, then add a bit for the up-haul halyard. Or any lines from the fore stay or back stay.
You could also get some ideas of scale of you do a photo search on google, if you know the name of the model. Or a rough idea of ratios by looking at photos of other pond yachts. When comparing photos, you should also compare keel types, some have small keels and relatively short masts. Others long keels and long masts.
Other than that, the rest of your thinking seems logical. If the mast is too tall, you will carry to much canvas and the boat may heel more than you want it too.