things to condsider in small embedded wearable applications

space usage
- microsd or flash
- bga or lqfp or other packages
- how many sensors
- are there sensors with multiple functions in one package
- communication with other devices
- battery placement and size
- how do-able will it be pcb wise

power
- usb or custom charging
- how long will it run on a single charge

programming
- how will one program the project
- will it support over the air updates
- can users modify the software
- how will you use low power modes
- how hard will it be to code it

misc
- how much will it cost (parts cost * 2,5)
- do others want it for the said price above
- how much of an interest is there
- is a kickstarter needed

at all times: think realistic!

so thinking you can fit an arduino zero/m0, oled, 500mah lipo, bluetooth, buttons, accelerometer, rtc and what not in just 40*40*10mm?
then you're wrong.

i just put this up for your reference.
i made quite the mistakes in the past without these own made guidelines