Effect of message length on collision attacks
The short answer is that for any secure hash it is impossible to generate collisions or second preimages, regardless of message length.

If you look at theoretical attacks, however, message length can be a factor. When looking for another message that hashes to the same value as a specific message you are talking about second preimage resistance (or target collision resistance) and there the length of the target message may matter. For example, with Merkle-Damgård hash functions there are attacks that find second preimages for messages faster the longer they are.

With a collision attack, where you look for any pair of messages hashing to the same value, I do not know of a case where message length matters. Beyond allowing enough input length to exhibit collisions, of course: finding e.g. 1-byte collisions is impossible.