More likely as not your unit has a rated output of about 65,000 to 70,000 BTU/hr.
HOWEVER, if you are planning on replacing this furnace do not use this figure for the replacement furnace. You really need a heat loss calculation for your house. You can do this yourself, just Google "heat loss calculation" (without the "") and there will be lots of hits. Most of the programs will have a cost but there are a few free ones available.
OR, call several furnace installers and ask if they will do a heat loss calculation.
One quick and dirty method to see if your present furnace is oversized (it probably is) is to determine its cycle time during the coldest weather. A properly sized furnace will run almost constantly during the coldest weather, at least 90% of the time. If your furnace is running only a short time then it is oversized.