First click Reports icon on the left sidebar of 6Storage dashboard.

You'll be directed to the reports page. Under Unit Reports category, you'll find Vacant unit report. Cick on it.

Select the location and building and then click search. The report data will be displayed below. You can also download the report by clicking on the pdf/excel icon. 

Scroll down. You'll see a secondary table with unit measurement and vacant units. Here you'll find the no of vacant units with respect to the unit measurements. 

Too many data on the report? Click the wheel icon on the right corner and choose your preferred data and hide the rest. To hide a column, you'll have to click on the checkbox.