The only property of a salt that is important from a security perspective is that is be globally unique. The length may impact how unique the salt can be, but is irrelevant from any other perspective. Assuming that it has a positive or negative effect on anything is to ask the salt to perform a function that it was never intended to serve.
So, a salt should be long enough to reasonably assure uniqueness, and that is all. Assuming your salt has good uniqueness, any longer will not impact security positively or negatively, but will most assuredly waste storage space.