I want to assign a touch sensor to the outer surface of a mash(whole front part of a robot). Is there is a way to define a site which covers the outer surface of my mash?
There is no automated way to do this, so you will have to adjust the site position, orientation and size manually. The site shape can be box or ellipsoid. You can visualize the site volume in simulate. The touch sensor will respond to any contact points that fall withing the site volume (or more precisely, contact points whose force vectors intersect the site volume). Here is an example from the documentation where site volumes have been adjusted so as to cover the fingerpads of a prosthetic hand model: http://www.mujoco.org/book/haptix.html#mpSensors