I figured out how to get a consistent size for single cycle waveforms, and it’s really simple.
Extract your waveform
Repitch it to 0.01 seconds long
This will result in an audio file that’s 480 samples long (for 48khz), which according to this calculator will be tuned to G4 + 35 cents. Using this approach you can build up a library of single-cycle waveforms that work perfectly for building wavetables.
TwistedWave is an awesome audio editor, and it supports this workflow neatly with its “Change pitch and speed” dialog: