Im just gonna use the greddy water pulley. What size belt do you think i need?

 

You will need a 5 rib which equals K050, 6 rib equals K060 and so on.....
So thats the first part of the part # you need. Now just measure the distance around the pulleys, one of those cloth rulers (like the kind people measure there waist with) or even a piece of string, however take into account that the measurement you get from the string will be slightly short due to it falling in the grooves of the pulleys.
So lets say it measures 36.5 inches, that would equal 365. know put that with the first part and you get KO50365.
Heres is a few examples...
4 rib 42 inches long = K040420
5 rib 29.5 iches long = K050295
6 rib 38 inches long = K060380

All parts stores use these part #s