The National Motor Freight Traffic Association (NMFTA) annually publishes a list of freight class designations, codes, and subclasses for commonly shipped items (https://classit.nmfta.org/). To access this list, you are required to pay a subscription fee. Learn more about freight classes for free on our freight classes pages.
If the couch you are shipping has not been assembled yet and you are shipping various components that you can use freight class 250. If you are shipping a complete product then you should use freight class 175.
Some items, such as car transmissions, have a specific class no matter how heavy or big they are. This is called a fixed class. Items can be assigned a fixed class if the value and transportability very rarely change.