Hello,
0.3A / phase @ 3.6V * 2 motors = 4.3W
Add 0.3A @ 5V for the CPU = 1.5W
total = 5.8... In theory 
Better have a little bit of margin.. so let us say 7-10W (USB typically gives at least 7.5W)
Thanks for all the footprint info/replies!
Cyrille