You need to edit the aircraft.cfg file, look for something along the lines of;
fuel_type = 1
number_of_tank_selectors = 1
Center1 = 0.4, 0, 1, 114.4, 0
LeftMain = 0, -6, -1, 16.8, 0
RightMain = 0, 6, -1, 16.8, 0
You'll want to increase the size of the larger numbers (114.4 or 16.8 )

Ozzy