Most people, including mathemeticians, just cannot mentally grasp how much a trillion of anything is.
At the current price of around $1,650 per ounce the $1 Trillion coin would:
Weigh 18,900 TONS.
Be a 30.4 feet by 30.4 feet by 30.4 feet cube i.e. 28,209 cubic feet
The gubmint, however, would produce a one-ounce platinum coin and stamp its value on the face as $1 Trillion. They may use lead since it approximates the color of platinum.