Not “small”, but I vertically mounted a 4u chassis (front-side down so cool air comes in from the bottom and hot air is exhausted out the top) and it actually feels not much bigger than a standard ATX tower, except I have 9 HD bays.

The case was $120CAD, and the mobo/cpu/ram (i3) were another maybe 300.

Caveat: I’m not running ZFS, and just do classic “data lives in two places on-site and one off-site” backup