You can edit the dimensions of a light fixture on its Basic property page.
To edit the size of a light fixture:
Select the light fixture whose properties you want to edit.
Right-click and select Properties, or select Edit > Modify Elements > Properties.
Select the Basic tab. Size properties will vary depending on the type of fixture. Use the diagram as a guide when determining the fixture’s properties. Clicking a dimension marked with an alphabetical character (a, b, c, etc.) highlights the corresponding dimension in the light fixture diagram, and vice versa.
Click OK.