With help of many silly height comparison tools, I got the size difference I wanted. Except for Micro, I can’t figure out how small I can make them and keep it within the size of a “working mammal”. I liked my initial idea of Micro being small enough to sit comfortably on Standard’s hoof.
XXL standing at 55 centimeters from top of the head to the ground
Standards standing at 23 - 28 cm
Micro standing possibly at 4 cm
I just realized how FRICKEN CHOMKY I made them 
So the smallest mammal is the Etruscan shrew, which has a body length of 4 cm.
That said, horses and other larger quadrapeds are measured at the shoulder, so a micro standing at 4 cm tall with the same body height/length ratio as a normal horse (5ft:8ft) would be 6.4 cm long, nose to tail.
This makes them about the same size as a chonky dwarf hamster (dwarf hamsters are a bit longer, but they’re not as tall):
If you made them 4 cm long to match the shrew, then they’d be 2.5 cm tall, which about the same size as the shrew I think. At that size, keeping the fluffy body shape, they’d be primarily herbivores as pretty much anything could munch on them, from ants to cockroaches to mice.
Love this take with dwarf hamster, thank you so much for the suggestion! Those gonna be some fat pighorse hammonds 
My daughter had hamsters (first was a Syrian called Cookie, the next two were a pair of Russian Dwarf sisters called Cherry and Strawberry), so I’m very familiar with their sizes.
Both types of hammie would quite happily chow down on a micro’s babbehs if they were anything like the ones my daughter kept, although I think an adult micro would be too big for their prey range.
How cute would it be for a hamster to have his cheeks full of little micro foals?! Awww.
