Dean - AN flexible hoses and fittings were developed many years after rigid tubing to provide us another alternative. Rigid tubing sizes were already standardized by the tubing outside diameter. As Paul wrote, the system for AN numbers are based on sixteenths of an inch.
To answer your question, AN hose sizes are designed to match the inside dimension flow rate of rigid tubing. There lies the confusion, rigid tubing measured outside, AN flexible tubing measures inside flow.
SIZE O.D. I.D
6AN
.3/8
0.34
8AN
.1/2
0.44
10AN
..5/8
..0.56
12AN
..3/4
..0.69
16AN
..1
0.88
Before further designing your main fuel feed line and parts, you may want to read Spuds FIE article on the Perfect Fuel Tank especially the EXIT fuel line segment. You may come to conclusion a -20AN line too large for your application and may cause other issues. Alan