I can vouch for the D1 mini also. It's the one you want, by far. It has an on board programmer so you can power and program it from USB, while being the smallest board in size out of the popular ones.
Have you tried the NodeMCU? (Not suggesting it, as I actually killed mine when trying to program it, but more curious about whether you’ve compared it to the Wemos D1.)
The actual smallest I've seen is one I made: https://github.com/skorokithakis/tiny-ESP8266-breakout