I've brought a touchscreen, the Adafruit PiTFT Plus 320x240 2.8" TFT + Capacitive:

https://shop.pimoroni.com/products/adafruit-pitft-plus-320x240-2-8-tft-capacitive-touchscreen?variant=31759720842&gclid=EAIaIQobChMI_6LKo7mp4AIVhuR3Ch2U_gdgEAQYAiABEgK2fPD_BwE

2.8" is large, especially for a watch style device. But that's a main factor for my project, I feel the small screens that are typical of a smartwatch really hinder their functionality and acceptance. The interest and talk around the new Apple Watch Series 4 and its slightly larger screen has had numerous comments on how that as helped with usability. 

2.8" I think is the largest I can go within reason.