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.
"Proper effort is not the effort to make something particular happen. It is the effort to be aware and awake each moment." - Ajahn Chah
"When we see beyond self, we no longer cling to happiness. When we stop clinging, we can begin to be happy." - Ajahn Chah
"Know and watch your heart. It’s pure but emotions come to colour it." — Ajahn Chah