All I did was measure to the height of my horse's chest (mine are both 16hh TBs) and that gave me the height to cut the pallets down to. I'd use the shortest of your minis as your maximum then.
The purpose of the hay-ring is essentially to keep the animal from 'barging' into the round or bale of hay... the "good stuff" is in the middle and they will tear the bale apart getting to it. They will then foul the perfectly good, but less yummy hay that got in their way and the bale won't last as long as it could've.
If your minis can't reach the middle of the ring then, every so often, you'll just have to rake the middle towards the edges.