Which board is the best fit for my project?
For size and very low thermal profile, look at the FemtoDuino:



The new design has on-board USB. Also, the board is so small that pressure / rough use stresses are not a big concern. 

However, the headers are not the standard 0.1 inch type (obviously), in order to fit everything into that tiny form factor. This isn't an issue for a custom project, I suggest.

There is also the LilyTiny, quite small, and specifically designed as a wearable Arduino like the LilyPad, but with limited functionality compared to the FemtoDuino. It is, however, insufficiently specified for the sensors the question mentions - not enough pins exposed.