How do I figure out the dimensions of each plane of 3D object?

Hello! I need help! I am trying to essentially wrap this graphic around the bag to give effect of black on black. The bag would be matte black, while the graphic would be glossy. I'd also have variations of the bag so the side panels would be bold contrasting colors, while the front remains the traditional black on black print. So I want to format the graphic to the correct dimensions so it is proportional to each plane.