Arduinos is one of the great examples of open source. The open license of the product allows for a variety of form factors to be available. As much as it is recommended, I always found the Uno model to be big. I started messing around the pro mini series, a much smaller form factor and fit most of my project ideas. For projects requiring even smaller size, I figured I would eventually leverage a PCB service to make a super lean form factor. Or find someone else who has done all the work.
The pro mini has been my trusty and comfortable size, as it is much smaller than its larger siblings and I am able to fit it into almost anything(e.g. gloves, caps, medals).
With the Femtoduino, I can realize a larger range of projects(e.g. necklaces, watches, maybe even goggles). I will definitely have fun with this!