The tank has a huge "dogleg" in it, I wouldn't be surprised if it held at least half a gallon. It also gurgles and burbles and spits fuel back.
FWIW I always top off my tank and haven't had any problems, but I may now stop at the second or third "click." Some gas stations have very sensitive "shut off" mechanisms (designed to prevent fuel spills, I'm sure.)
At my preferred station I can typically go at least 1 - 1.5 gallons after the first click, so I know the tank's not full. In some stations, from a nearly empty tank, the fuel shuts off at 13.5 gallons and if I'm careful I can put another 2.5 gallons in.
Also last time I filled I put about 18.2 gallons in.
The most I've ever put in. What happened was that the low fuel light came on, as usual, around 260 miles and I ignored it. Drove around for several days and after I got to around 300 I finally chickened out and put in about 2 gallons. Drove for another 30 miles or so (with the low fuel light on the whole time) and then filled it with the 18.2 gallons.
I'll never let it get that low again.