If the Small Package Quotes application is installed, you should enter dimensions on the product shipping parameters page so that:
The Small Package Quotes product has a proprietary packaging algorithm that identifies the optimal packaging solution required to ship the order. There is also an optional standard box sizes feature that will determine the order's packaging solution using your standard box sizes. In both cases product dimensions must be present for a packaging solution to be identified. When dimensions aren't available rate estimates are retrieved using weight only and may not be as accurate.
The packaging solution identifies the boxes required to ship the order, and each package's (packed box) weight and dimensions. This information is used to identify each package's billable weight. Parcel carriers define the billable weight of a package as the greater of its actual or dimensional weight. Actual weight is the weight of the package rounded up to the next whole pound. A package's dimensional weight is determined by multiplying its length, width and height (round each measurement up to the next whole inch) and dividing it by 139. Round the result up to the next whole number to arrive at the dimensional weight in pounds.
Illustration from the 2018 UPS Rate and Service Guide
Entering dimensions for your products improves the accuracy of the parcel rate estimates displayed to your shoppers.