A raspberry will do fine as long as you do not expect huge speeds.
The CPU is not capable of pushing data very fast.
Expect somewhere around 0,5 Mbyte/s (4 Mbit/s).
If you go smaller and cheaper you will not reach even that speed.
So it depends on what kind of speeds you are after.
A relay does not need a lot of attention. Just make sure that you update the OS and Tor every now and then.
There are plenty of guides on how to do it which can be found by googling.
This is a good example which has a link to Atlas with some relays running on Rpi:
https://github.com/DFRI/dfri-rpi-tor
From: tor-relays <tor-relays-bounces@lists.torproject.org> on behalf of Tamara West <sinister.hama@googlemail.com>
Sent: 16 October 2016 21:22
To: tor-relays@lists.torproject.org
Subject: [tor-relays] Smallest, cheapest, lightest computer for tor relay
I've got a few family members across the country who have broadband they are not using for more than email and social. I wanted to go about setting up a relay at each house can can be run with minimum amount of power and attention. I was considering
something like RaspPi or Compute Stick.
Main questions (1) anyone else use these or something similar? (2) recommend of strong alternatives suggested? (3) is there something smaller, lighter and cheaper that will do the job? (*) did I maybe miss something obvious that needs attention?
Thank you.