It’s hard to say. You can host a frontend app with 256MB of RAM and the lowest CPU tier, but if your customers intend to do dev tasks on it, say transpiling TypeScript, you’ll probably need something beefier.
I’d say start with the lowest tier, and then bump up the RAM/CPU until it performs to your users’ liking. There are plenty of resource combinations to choose from.
I’m currently going with a 2GB shared cpu instance, which seems to work quite well. The performance cpu seems to be much faster, but I think this is the best choice for now given the much lower price.