When you come up with something and test your model out, you can use conductivity as a general measurement of how pure your water is. Easy to measure, and once ppm dissolved solids is measured also easy to correlate. To shrink your design, you need some sort of forced condenser section; a section to condense the water after you have boiled it off. In chemistry, you run water through a piece of glassware to cool it. I think a solar still is much more efficient in terms of getting purer water (evaporation vs boiling). You may find how drinkable your water is depends upon your design. You may be stuck with running the water through twice to end up with drinkable water.