Product Development using ARM mBED OS
ARM mBED is just a software library with a easy to use abstraction layer of typical micro-controller peripherals (SPI, GPIO etc.). It is not even a very complicated one. 

Being a software library is has no impact on the form-factor of your final product. 

There aren't really any hardware requirements except picking an ARM chip that is supported by mBED. In the end, if your software is working all what's really required to run the mBED code is the micro-controller, a voltage regulator, a hand full of bypass capacitors (and maybe a crystal oscillator if you need precise timing) plus whatever peripheral chips you interface with.

So there is not a big difference to a bare metal system.