The smallest HID spotlight at 7″ with reinforced mounting brackets. However it is 55W which is a fair improvement over the 35W hand held spotties.
PRO9 HID $550
The most commonly sold roof mounted spotlight is this 9″ HID 55W. Reinforced brackets.
PRO11 HID $650
With a reinforced bracket, this is the biggest you can buy at a massive 11″ across. Again it’s a powerful 55W to really stretch out there and see something.