You can deliberately use long breaths for a while if you want if you need to help the mind calm down.
Otherwise you just let the breathing happen naturally.
If you are doing a mahasi style noting technique then it might be approprate to label the breath as long or short. Otherwise just noticing whether it's relatively long or short is enough and you don't need to make a big thing out of the longness or shortness, it's just one aspect that you might notice.
Some teachers do encourage a progression of longness and shortness though, See Anapanasati by Buddhadasa Bhikku, or some of Ajahn Brahms explanations.
"Whenever we feel that we are definitely right, so much so that we refuse to open up to anything or anybody else, right there we are wrong. It becomes wrong view. When suffering arises, where does it arise from? The cause is wrong view, the fruit of that being suffering. If it was right view it wouldn't cause suffering." - Ajahn Chah
"Remember you dont meditate to get anything, but to get rid of things. We do it, not with desire, but with letting go. If you want anything, you wont find it." - Ajahn Chah