To be fair, Arnold was in pretty good shape in T2 - certainly not as big as T1, that's for sure despite his claims, but in great shape nontheless.
His bodybuilding profile shows:
Height: 6'2"
Arms: 22"
Chest: 57"
Waist: 34"
Thighs: 28.5"
Calves: 20"
Off Season Weight: Around 235 lbs.
Competition Weight: Around 260 lbs.
I don't know how applicable these stats are for T2 Arnie, but perhaps they wold be a good starting point (or maybe go with slightly smaller stats).