this may be very basic, but how should I handle products with more than 1 model?

For example, I sell desks. It's the same desk design, but one is 200cm long and the second one is 170cm long. Obviously, their prices aren't the same, so I need to specify in the quote to my client the lenght and therefore the change in price.

I don't want to create two separate products, since it is the same design. It's the same product with model A and model B.

Thanks a lot for the help.

Rod.