It's not that the size doesn't matter; it's just that it's not as significant. It becomes very, very hard to compute rainbow tables after just a few random bytes. No matter how long the salt is, it doesn't do anything to prevent somebody from trying to guess the original input for a given digest using a brute force approach, though. So usually the salt has less than e.g. 256 bits of entropy just because it takes up less space.
Sure, a very large salt might slow down the first iteration a little (but not necessarily subsequent ones, and it wouldn't require more memory, at least with most hash functions), so you're almost always better off just stretching the key--then you save the storage costs too.
Sure, a very large salt might slow down the first iteration a little (but not necessarily subsequent ones, and it wouldn't require more memory, at least with most hash functions), so you're almost always better off just stretching the key--then you save the storage costs too.