To ship large items which are not very heavy, freight companies normally charge by volume.
To calculate a weight for these products, freight companies have special formulas, which allow calculating a volume weight to enter for a product.
This weight will not reflect the true weight of the product, but will allow for its size to permit the calculation of the shipping charges which will be applied to it.
Your freight company will be able to let you know, which formula to apply to your large products to calculate a suitable weight.
You can also set up extra charges for specific product only by using the Product shipping / handling costs function.