If you want to truly minimize the size of the package, the best way to do it is to
- physically build a prototype, and
- use a power meter to measure the consumption under worst-case* conditions.
*This will be a function of whatever hardware you're using and (perhaps more importantly) the computational load. You'll have to find out how much load your software suite will put on the system and ensure that your test suite meets or exceeds this load.
I would add a 50% derating factor to your measurement to account for unexpected things - you don't want your embedded system browning out after doing a software update, for instance. This also will improve the life of the power supply (less power = less heat = longer life).