I just replaced the 18 gal tank in my '00 wagon.
Bone dry 5 gal. of gas put the gauge a hair over 1/4 on the gauge.
Remember the gauge is not a precise measuring device, just a approx. indicator.
That's why running the tank lower than a 1/4 on a regular basis starves the pump on corners and shortens it's life. Fuels not that deep in the bottom at that level to keep the pump intake covered.
I would say in my car 16 - 17 gals is the max usable until your walking.