Hey there! This is one of those questions where there is never a good answer. What size pizza should I get? Well...that depends on a few things.
It's important to note that we don't have separate hardware requirements for the DNSOnly product, so you'd need to follow the hardware installation requirements here:
This document describes the system requirements for your cPanel & WHM installation.
docs.cpanel.net
If you want more than that, it's up to you and how busy you think the system will be. Is it handling the DNS for many active sites, or just a few sites with not much traffic. How many domains will it be serving?
With modern VPS solutions it's often easy to scale the RAM, so you could always pick a number and them adjust it up or down as needed - for example, if you're seeing high RAM use with 2G you could bump the machine to 4G after a trial period.