It is not quite as compact as the picture makes it seem.
The size is 75mm(W) x 90mm(H) x 19mm(D)or about 3"(W) x 3.5"(H) x 0.75"(D).
It is still pretty compact, but not quite wristwatch size.

It seems to be a pretty stripped down unit so it should be simple.
I like the idea and will consider buying one when my old e-trek wears out.