The best way to get a measurement for your carving is to measure the fish in the position you want it.
Example: true total length is with mouth closed and tail pinched down, however your mount will never have a pinched tail up on the wall.
Typically a girth measurement is for the widest part of the fish.
The more girth measurements you can get at known locations the more realistic your carving will be to your trophy.
Fish carvings can be done in any style. A one sided carving can be mounted onto a base for the wall. A full body carving can be mounted to a table top base.