I remember trying to do a similar project for the bus stop at the corner of my street to know when to leave to catch the bus. It was fun, but I never really finished when I realised how big just the Pi would be (still powered by usb-a, etc).
Nice to see this one so well executed!
Regarding the size of B series, unless I need the ports, I almost always snip out the ports — don’t bother desoldering it, the ground plane in Pi acts as a massive heat sink. Instead, I snip slowly and carefully the metal housing of ports, and then wiggle the plastic bits until the break away. Reduces the size, and leaves sufficient space to put small modules in place.
I posted a few pictures in my previous comment here: https://news.ycombinator.com/item?id=20462711