You can view the Dimension Names form from the Inventory tab of the Company Preference form by clicking Dimension Names button. The Dimension Names form allows you to modify the 3 fields, namely Length, Width, and Height that will be displayed in the Unit of Measure area of Inventory Items form - Setup tab.
The OK button will close and save the Dimension Names form.
Press F1 or click F1-Help button at the bottom left corner of the form for help on what this form is all about.