Where can I add the weight and dimensions to a product on Shopify for shipping big items?

Shopify store owners who ship their own product with calculated rates by carrier or app, help!

I’m trying out the shipping option that charges the appropriate rate based on the weight and dimensions of the product, but did I miss something? In my inventory, I only have the option to add the weight of the product. This is a huge issue for my oversized items, and I don’t want to add a 120% handling charge to the rate to hopefully make up for the shipping of big items (example, 30”x22”x15” 12lb parcel) and possibly scare away everyone else.

Where do I add the dimensions for a product so that Shopify gives the customer the most realistic rate?

Is my theme missing something everyone else has?