Sinatra is great - it's small enough to read the whole thing and understand it. But it's also very minimalist, so you might also want to look at Padrino, which layers neatly on top of Sinatra, yet is still small and ORM agnostic.

It's also fairly painless to start with Sinatra, and layer in components from Padrino if/when you need them. E.g. you can simply register Padrino's helpers, mailer, routing, rendering or cache components in a Sinatra app if/when you need them. They're just straightforward Sinatra plugins.