Two variables need to equal a product

I need to construct a train cart.

The train must have a volume of $\displaystyle 1.25 m^3$

As a train cart is on tracks, the width is set to 0.8 meters and cannot change obviously.

That leaves the length and height of the cart to be the only variables we can change:

$\displaystyle (0.8 * a * b)m = 1.25 m^3$

A quick reduction where 0.8 is divided on both sides gives:

$\displaystyle (a * b)m = 1.5625 m^2$

So, now my question is, how do I decide what the two variables (the length and height of the cart) should be?

And there is one more last thing to the question. Since the material for the cart is very expensive, you must use as little material (the surface areal of the cart) as possible. The cart is obviously open at the top of the cart.