The answer was 2 feet.
What is your source? How much do you regard as "whistles" of the arrangement?
Some figures: From base base of the branch to top of the center dome: 4 feet 2inch
Width across the outer bells: 3 feet 6 inch
Diameters of the 3 bells: 9 15 and 12 inches.
But I have not found any measurement of the bells and there were 3 different sizes.