There are a few old retro fish templates floating around on Sways;
From the Kinstle book (page 61)
From Greenlight Surf Supply
From Harbour Surfboards on the Fish
“to remain true to their design concept it is recommended to stay within the 5-6 and 6-6 size range. These boards are for waves in the waist to head high range, although this does not restrict someone from pushing these numbers.”