TVTropes has two different entries on Mini-Mecha and Powered Armor, where the former is "something that is piloted" and the latter is "something that is worn". Now, listed in both pages are examples of things which are supposedly halfway between the two concepts, but most of them have no size estimate. So, what is the biggest powered armor / smallest mecha (meant to be used by average-sized humans) that you can think of in hard sci-fi? Is there anything about 8 feet / 2.5 m tall?
From the top of my mind:
- The working suit in Aliens is about 3 m tall.
- The armor of the marines and other infantry in StarCraft is about 2.5 m tall.
I've invented a new criterion for separating them: if the person's fingers fit inside the machine fingers, it is a powered armor. if it has fully robotic fingers or not fingers at all, it is a mecha/mini-mecha.
With that in mind, both of my previous examples are mini-mechas. Generally speaking, any machine of more than 2.5 m will have independent fingers, or else it will need to have very short arms -in proportion- for the user to reach the mechanic hands.