Your chain size can be found online or in your manual for your bike. The size will usually be something like "525-100"
The first number is the chain size. There only a hand full of chain sizes. For these older bikes its usually a 520,525,or 530.
The second number is the amount of links. The amount of links should also be in your manual. worst case scenario you could count them but the information should be pretty easy to find. Keep in mind that if you are increasing sprocket size you will need a larger chain. Also you can always take links out but its hard to add links you dont have..when in doubt, order bigger.