On my lil longtail it had a key in the keyway that's like 3 inches it was a bit to get off I'm about to reassemble it and I'm wondering do I need a key this long or can I cut it shorter I don't want it flying out but don't wanna spend 2 hours getting out again lol all opinions welcome pic for reference

As long as the key is long enough to cover the area where the slip yoke fits onto the shaft that's all you need. It's not necessary for the entire key way to be full. You probably need about half of what you have pictured there.