1991 Lincoln Town Car Serpintine belt

If you mean the water pump is not going around, the problem is not the serpintine belt--it is the water pump. If you mean the serpintine belt is not going around the water pump then the engine will overheat.
The size of the serpintine belts differs from car to car. If you go to the auto supply store and tell them the engine size and whether or not it has air conditioning they will have the right belt for you. If you must measure it (I don't know why you would) but -- with the engine off get a rope or ribbon about 8 feet long and weave it through the serpintine belt pattern -- this pattern should be diagramed somewhere under the hood--if not obtain a repair manual for your car.
Serpentine belts that show signs of cracking and wear should be replaced. However I think the squeeling is caused by something more than just the belt.

the belt on the 91 is a different length depending on whether it has factory air or a/c installed by the dealer. The vin # should show which it has.

How difficult is the tensioner to replace on this car?

I'm told this is a mid year model and changes were made to the belt system and an adapter is needed. I can't find this info anywhere but a Ford parts dept.