The best tire height would be one that does not exceed factory width/height so you won't run into any fitment issues. There are several online tire size calculators you can use to enter your current size then play around with the 20" sizes to get the one that matches closest in width and diameter.
It's a pretty important piece, I agree that taking it back and telling them it fell off your car after the repair was made and they need to find out where it goes and re-install it.