While the gas tank is advertised as 6.6 gallons, that includes necessary head space for venting and expansion. The usable capacity is 6.0 gallons. The low fuel light comes on with about 1.5 gallons left, so at that point you should be able to add around 4.5 gallons.
There are some tricks to filling the tank fully, without overflowing to the emissions canister. First, unlike a car where you fully insert the nozzle into the tank, on the Spyder you have to insert the end just past the baffle, and hold it there. In areas with emissions collars on the nozzles or with high nozzle vents, this may be difficult. Fill manually, don't use the autoatic shutoff feature of the nozzle. Fill slowly, especially toward the top. Filling fast can overflow fuel to the evap canister. Stop filling when gas begins to spurt out the vent slots in the fill baffle. You can do another try to be sure the gas is full, but stop for sure the second time you see gas splashing through the vents. Any more will overfill the tank. Using this method consistently should allow you to fill the tank to its 6.0 gallon capacity each time.