Well, I'd say it should be big enough to make Gorilla look like a baby when comparing it to the Golem.
The Golem's shape should make it look extremely strong when comparing it to other machines. Its arms should be long and big, so when that Golem swings its arm to kill all the units in its range, you feel like its arm is not an arm, but a giant broadsword, penetrating everything on its way.
The Golem should also have some special weapons, allowing it to punish its enemies (but they shouldn't be overpowered, otherwise Golem could always win):
- Some missile launchers slowly firing slow but deadly rockets with huge AoE.
- A few rapid, low-damage machineguns capable of killing single enemies with ease.
And, of course, his arm - broadsword-like weapon, destroying hordes of enemies in no time, but with limited range (Gorillas can survive a hit or two, though, guess why).