I have seen the TinyScreen Smart Watch Kit, but I consider it "for educational purposes only". No one would go around wearing a huge watch like this. There is so much space taken by the PCBs of the shields, in length, width and height (empty space between the shields), and also by the connectors.
So, I was thinking, could you design and build a board that has all the necessary components to make a smart watch? You don't even need to have the USB interface chip and USB plug on the watch, the programmer could be a auxiliary programming board or it could be an arduino, as it will be used rarely. For example, there is the RFduino, smaller board, with bluetooth, no usb. (I didn't check the price, I just give an example).
Hopefully, all the components could fit on one board under the OLED screen (together with the screen driver chip).
Now, to make a watch with many features, it would be nice to include at least an accelerometer chip, if not heart rate sensor, blood pressure sensor etc. My main idea is to make a "vital signs smart watch" for my elderly mother...