There is lots of play so no worries.
Bathroom sink tailpiece too long.
If the tailpiece will not line up with the trap move the trap into its original position tighten the collar with the pipe wrench and obtain a flexible tailpiece extension for your drain.
You doing this for measuring purposes only.
Cut the extension to the proper length.
Then cut tail piece accordingly.
New tailpieces come quite long so they can be trimmed to fit.
Tail piece should be long enough to be fully inserted into p trap and seal.
I suggest that you do dry run meaning assemble and hand tighten all drain pieces including tail piece first.
Trim the tailpiece to fit.
A pipe cutter would be a bit more suitable than a hacksaw but if you only have a hacksaw just clean up well with a file after sawing.
Slide it onto the tailpiece and.