You could run a relay on a RasPi if you wanted. It just needs a basic webserver (nginx, caddy, apache) and a domain with SSL cert installed so connections over websocket secure (WSS) can be made.

Disk space and resources can depend on a couple factors including the lang that it was written in and the database. I’ve found the relay written in Rust with SQLite to be more performant and low-resource consumption than one written in Typescript and using Postgres.

And then it also depends on the type of NIPs the relay allows, which will dictate the sort of content stored on the relay. Also factor in number of connections using the relay, if it’s mirroring another relay, and/or if it supports hosted media like NIP-95.