Probably something like an n100 based "NUC" type deal. Its has loads of float performance and is much better suited to being a "server" than a pi (much as I love the pi)
If the goal is smallest VPN box instead of best for the price server then the float performance doesn't really matter much and both are probably overkill -> too large. Both the n100 and the pi 5 can reach multiple gbps of wireguard throughput, whatever you can get in the smaller total form factor is more ideal than ridiculous throughput.
