You might / will need a sidebag if you're going to get an iDSD BL, I wouldn't imagine it as a pocketable device. On this note, if you do get a sidebag, then whether it is a Mojo or an iDSD, you're having a sidebag, so the size is less of a concern, the smallest sidebags should work with it.
The only thing you need to consider is the sound, once you do hear the sound, it is quite hard to care about the size anymore
I should sped more time with HA-2 and Mojo in the future to have a better comparison about the sound, but for the moment, I am quite happy with iDSD BL and it still is the benchmark DAC/AMP I use