I bought a set of pulleys without belt for my FD and now I have no belt (forgot that part) so my question is what length does the belt need to have?

 

Depends on what you bought. Here is a quick reference

http://www.banzai-racing.com/products_FD_belts.htm

 

You can also put your alternator at it's lowest position. Then, take a string and run it around the path of the belt. Next, measure the string to see how long the belt path is and then find a belt local which is close to that length. A 4 or 5 rib belt will work.