If you are changing pulley sizes, but keeping the belt tensioning system stock, and also not altering pulley location, then this is for you.
The easy part: for every inch you change the pulley diameter, you need to change the belt length by 1.57 inches.
Example 1- My stock belt length is 95 inches. If I measured my stock pulley at 3 inches, and replaced it with a 5 inch pulley, the difference is +2 inches.
I need 1.57 * 2 = 3.14 inches extra belt length. I would go buy a 98 inch belt.
Example 2- My stock belt length is 95 inches. If I measured my stock rear pulley at 9 inches, and replaced it with a 6 inch pulley, the difference is -3 inches.
I need 1.57 * -3 = -4.71 inches shorter belt length. I would go buy a 90 inch belt. Round up or down depending on the decimal.
Why does this work? When you add 1 inch to the diameter of a circle, it adds 3.14 inches to the circumference. BUT, we are only using half of this circle! The belt only wraps around half of the pulley (generally speaking) so you only need half of the 3.14, which is 1.57 inches.
NOTE: When measuring a stock steel pulley, check to see where the belt rides in the groove. Most stock pulleys have a large lip to help keep the belt on. The belt usually rides about 0.5 inch inside. So a pulley measuring 10 inches in diameter from the edges might actually be smaller (9 inches: 0.5" on one side and 0.5" on the other side) because the belt rides inside further. Most aftermarket aluminum pulleys have no extra lip.
One last example- My stock belt length is 95 inches. If I measured my stock pulley at 3 inches, and replaced it with a 5 inch pulley, the difference is +2 inches. My stock rear pulley is 9 inches, and I replaced it with a 6 inch pulley, so the difference is -3 inches.
I need 1.57 * 2 = 3.14 inches.
I need 1.57 * -3 = -4.71 inches.
Add those two numbers together.
3.14 + -4.71 = -1.57 inches. This is the difference of the two pulleys. I would buy a belt that is 94 or 93 inches. Not perfect, but I would have to see what the store has in stock and try both if available. In my case, the store only had a 94 inch belt and it worked perfectly.