I've used Flask (and bottle.py) in the past, but these days I actually prefer Pyramid, from the Pylons folks.
Pyramid is capable of being a large, full-fledged framework, is designed for flexibility, and has no shortage of plugins and extensions available adding additional functionality -- but it also is capable of small, single-file projects; see this tutorial for an example.
Going with Pyramid will give you room to grow if your needs expand over time, while still retaining the ability to start small.