Should I cut the tailpiece of my new drain? It too long to fit the p-trap and waste line I have in place. Do I have to uninstall the drain to cut it or can I do it in place?

Trim it to desired length. Yes, you can cut it off in place. Back to you. Milo

I agree with Milo,
Cut the tailpiece to connect to the trap. However, it would be much better to remove it from the drain and cut it and would not disturb the connection to the drain. Good luck, Tom