How does the FAA define fuel capacity for ultralights?
FAA Advisory Circular AC 103-7, Paragraph 19 has this to say:


  MAXIMUM FUEL CAPACITY OF A POWERED ULTRALIGHT VEHICLE.
  
  The maximum fuel capacity for a powered ultralight vehicle is 5 U.S. 
  gallons. Any
  powered  ultralight with fuel tank(s) exceeding this capacity is
  ineligible for operation as an ultralight vehicle.  
  
  a. Determination of Fuel Capacity. 
  
  The total volume, including all  available space for
  usable and unusable fuel in the fuel tank or tanks on the  vehicle is
  the mtal fuel capacity. The fuel in the lines, pump, strainer, and 
  carburetor is not considered in a calculation of total volume.