Okay...28" long x (say a radius of) 6" x 3.14^2 = 1656.4128 cubic inches of blood, divided by 2 (because there's stuff inside a penis besides blood) = 828.8064 cu. in. of blood.

That's 14.34 US quarts (or 13.57 liters) of blood to engorge the penis. Since the adult male human body contains only about 5 US quarts of blood (or 4.73 liters), erection couldn't be accomplished without a bunch of friends to help out by pumping in some of their blood.

This, of course, assumes that the 28" is attached to a human being....