Hi!
from my knowledge, Mixamo auto-rigger should be able to correctly operate on characters of any size. But I can’t guarantee.
Retargeted UE4 animations are adapted to the size of the target skeleton at run-time, using the Translation Retargeting options defined on the destination character.
If all of your skeletons are retargeted toward the same skeleton (e.g. the UE4 mannequin one), then you can retarget the single “original” animation to all other skeletons (Reatarget Anim Assets > Duplicate Anim Assets and Retarget) and UE4 will automatically adapt the animation to the different scales. Please note that animation retargeting is a sort of “hack”, so you could notice issues if characters have sensible topological differences.