The tip should match the shaft end with a little material left for shaping. Too small exposes the edge; too large requires excess leather removal and can alter the profile.
Measure first
Use calipers on the bare shaft end after removing the old tip. Confirm the face is flat, then select a size from the maker's range.
Install carefully
Centre the tip, use compatible adhesive and shape it without cutting wood. Once cured, feel for a raised edge and make gentle test strokes.