I've made some spreadsheet work upon the battery size.
It appears, with 15W global power drained from the setup when transmitting, that a 3S2P should be enough and smaller to fit in the box (and also lighter).
Here is the screenshot :
So, more than 4 hours just on transmit is not bad at all !
If anybody have already measured the real power drain if the ubitx when transmitting (not the output power, the power used on the power supply).