We had a similar dilemma a couple of years or so ago when we began our migration to Meraki kit (so glad we did)
11 sites through-out europe ranging in size from 100 people to around 5 - plus host system at a data centre.
ended up with 2 MX84's for sites with 70 + users and rest went with MX64 and one Mx65 for site that was going to have dual circuits (but didn't in the end - doh! )
It's best to go have to much than not enough as you end up buying twice otherwise.