Help constructing parabolic bridge?

I have to make a blueprint to create a parabolic bridge. The bridge has to be scaled to a real life version of the bride in which a certical drop canyon is 550 yards deep and 110 yards wide. I am using a box to simulate the canyon walls and by box is 1 ft wide by 1 ft long...so that gives me a scale of 330:1 I believe...how would i go about finding the dimensions of the parabala and scaling it? does anyone have the equation or can anone help me?