A favorite question, often revisited.
Tinies under 3 inches can be swallowed whole, which may or may not be appropriate.
Enclosing little ladies in my hand or mouth is very important to me, so 3-4 inches is the maximum height for that consideration. That height also works for most pockets.
Between 4 and 6 inches is also pleasant because her features are easier to see.
Above 6 inches she becomes difficult to contain. I like a good chase as much as the next guy, but every day gets to be a chore.