Summoning an ArmorStand with custom NBT-data like so:
"summon ArmorStand ~ ~0.5 ~

"
creates a small ArmorStand. Its hitbox however is the same as the one of a large ArmorStand.
Interaction with the equipment-slots is scaled properly. Interacting with the upper half of the hitbox is the same as interacting with the top of the default ArmorStand which used to be associated with the hand-slot, but was changed to now be the helmet-slot.
I used "F3+b" to highlight the hitboxes on the screenshot.