To get accurate measurements, the hoof you measure needs to be clean and be freshly trimmed.
Measuring Width
Place your ruler on the 0 at the edge of the hoof wall at the widest part of the hoof. Your measurement will be from there to the edge of the hoof wall at the opposite side.
This hoof would measure approx. 115 wide.
The donkey hoof here will measure at 73mm or so wide
Measuring Length
To measure length place your ruler on 0 at the edge of the hoof wall at the toe. Your length measurement ends at the end of the frog as shown by the red line here.
This hoof would measure approx. 127mm long
The hoof in this image measures approx. 137mm long
Shoe application demo.
Trim hooves
Fit and modify shoe as needed
Weld on cuffs/tabs
Buffy the hooves clean and smooth
Apply hoof putty to collateral grooves and other areas that have or might grow fungus (white line or seedy toe)
Mix impression material if using
Apply copper sulphate to impression material
Apply super glue to cuffs/tabs
position impression material onto the bottom of the hoof mostly over the frog area.
position the shoe on the hoof and get the horse to gently set the hoof onto the ground and pick up opposite hoof, this sets the impression material into the hoof.
Wrap tightly with stretch plastic wrap.
Once impression material is set, remove the plastic wrap and apply super glue around the edges of the cuffs/tabs and over the exposed hoof. this helps to seal everything up.