What is the best plan to handle 100+ concurrent users?

Before trying a resize, my recommendation is to look into optimizing your database. We have a guide on tuning your MySQL which should be able to help you out with that:

MySQL Tuner

There's a specific section of the guide which might pertain the most to you, which I'll link you directly to here:

max_connections

If you do decide to resize, we have some documentation on picking the best plan for your purposes:

How to Choose a Linode Plan

I hope this helps get you pointed in the right direction!