When editing a gantry via the equipment editor in the 'Configure' window, the 'Edit Gantry' window gives you the following options. If you want to follow the full process for adding equipment to a structure, please see the following articles:
Height: Change the gantry height in metres. Changing this value will also affect your mast length(s). This value can only be changed using the up/down arrows to ensure your mast lengths are valid.
Length: Change the gantry length in metres. This value can only be changed using the up/down arrows to ensure your gantry length is valid.
Please note: 'Length' is not available when editing a portal structure. You can alternatively adjust the RECOS or reach of a mast in the 'Edit Support' window or use the 'Edit Structure' window to adjust the gantry length of a portal. Learn more here.