The difference mostly shows for high detailed textures.
I prefer for the horse to do them at 4000x4000 compressed at 8 (80 percent) in Photoshop on the coat, eye and mane/tail/fets.
All bump, displacement, transmaps and specular at 6 (60 percent) and 2000x2000 pxls.
Anything larger takes a long time to load and render.
On human skin you need larger maps for showing the details for close ups and on the horse for a close up.
I prefer for the horse to do them at 4000x4000 compressed at 8 (80 percent) in Photoshop on the coat, eye and mane/tail/fets.
All bump, displacement, transmaps and specular at 6 (60 percent) and 2000x2000 pxls.
Anything larger takes a long time to load and render.
On human skin you need larger maps for showing the details for close ups and on the horse for a close up.