I am doing a furnace change out. The old furnace is a Mars the model number is H125 and the serial number is 56X257. It has 125,000 Btu input. I need to know what size ton drive this model would have. Any help on this would be greatly appreciated.

What is a "drive size "? Never heard this term before. AC? If so, there is no particular BTU. You need to have a Manual J calculation done.