this is one of those things that was never really defined on a machine by machine basis. even in the novels, it varied a lot. one novel would have the Wasp with a cockpit barely bigger than the pilot, with their head and shoulders the only thibng inside the mech's head.. while another novel with a different author had a stinger (which by canon is just a copied Wasp) where the cockpit was wholly in the head and big enough for the entire pilot to fit in with room to spare.
you even had things like a Mackie cockpit being big enough not only for a pilot and passenger, but having a storage closet big enough to house another person. (who in that book, was carrying a gun and wearing body armor, to kill Mechwarrior Horse if he chickened out of a one sided trial. it was a weird book)
even the same author was often inconsistent within their own novel. in Mercenaries Star a shadow hawk cockpit/head is big enough to fit pilot and a passenger in a rumble seat.. but the much larger marauder wasn't big enough for two people. later book by that author you had a pair of Grey Death/Nighthawk PA (it is unclear which) being able to fit into a single light industrial mech's cockpit with no problem.
so ultimately it boils down to "no one knows" and "it is up to you as the player/GM"