Call me old fashioned, but Shrink Rays all the way! Potions will do too, in a high fantasy setting.
I would also add natural/innate shrinking abilities. Either fully controlled, ie. they can change size at will without too much downside or cooldown. Or, the ability isn’t under the person’s control and they shrink either randomly(!) or under some external factors (emotions, getting wet/hot/cold, etc)