I want to build my own smartwatch with rasbipan, and i want to know if there exist raspberry pis or ardiunos small enough to fit on someones wrist.the biggesr issue for me is height.
Yes, there are ;) - and 3D-printed parts are pretty light.
The limiting factor is the battery. The Raspberry Pi (no matter which model) draws too much power to be usable as a smartwatch with a light battery.
A slightly smaller smartwatch using a Pi Zero: