Girth is the widest part of the fish in the middle of the fish. On a mainly flat fish it would be a little less than 1/2 the girth 17.75" divided by 2 = 8.875"
I added a link to the original post with pics.
This math only works if crappies are 2 dimensional. A fish that size is easily 2-3 inches thick (3rd dimension). A more accurate calculation would be
[(17.75") - (6.28")] ÷ 2 + (1" + 1") = ~7.74" "tall"
[(Girth) - (pi x 2" thickness)] ÷ 2 sides of fish + radius of back arch + radius of belly arch = approximate height from belly to back.
An 8" hole will fit a world record crappie.
Disclaimer: this calculation, while more accurate, is still imperfect due to variance in crappie thickness and shape of back and belly arcs not being perfect half circles.
Disclaimer #2: I'm not super fresh on my order of operation so my equation structure semantics may be slightly incorrect, the actual math is correct.
Edit #1: removed arc diameter from equation explanation.