guess work and pass the cost on to the customer. The Nissan fuel line is a rediculous price per foot. TTers have used the Goodyear FI hose some have had problems(?)
The 5/16 ID worked for me I got it at AutoZone. See my recent post on the relative sizes for OEM vs Goodyear. Reuse your OEM clamps, do not use the spiral screw type as they will damage the outside of the hose. You will need to retighten the clamps after awhile.