In the immortal words of Popeye:

I yam what I yam, and I'm strong to the finich 'cause I eat my spinach.

Eat well, exercise regularly, and enjoy the bushwhack regardless of height/weight.  There may be an ideal body type for bushwhacking but I suspect not.  There are tradeoffs any way you look at it.  Taller/stronger excels in some situations, holds you back in others.  Small and nimble also can be an advantage or a disadvantage depending on terrain.

I think when I am choosing companions for a bushwhack I'll be focusing far more on mental makeup and on how our personalities mesh than on body type.  Mental toughness or lack thereof is far more indicative of the ability to successfully negotiate a difficult bushwhack/trip than body type...presuming you're in reasonably good shape.  Personality counts for a lot too, because the stress level is going to go way up and magnify any personal differences to an extreme.