L = 2W + 2D - 8R + 2(pi)R
L = total length of tubing needed (single piece)
W = width of rack deck
D = depth of rack deck
R = radius of your tube bender
pi = ancient greek pie, 3.14...
or, simplified even further
L = 2W + 2D - 1.72R
If I'm right, this means that, starting with a 48-inch length of tubing and a bender with a 15/16-inch radius, the max depth you can bend a single piece of tubing into the perimeter of a 14-inch wide rack deck is about 10.8 inches (measured along the tubing's centerline).
48 inches (aka 4 feet) is the magic number for tubing; I've been told that pieces longer than this trigger increased shipping charges.
UPDATE: There's more info on this in the comment thread at Alistair's photo.