Log onto your webmail account.
On your Dashboard, click on 'Files' on the left-hand side of the webpage.
Now you need to select the files you want to calculate the size of. Click the square box beside the files you want to find the size of.
Near the top of the page, click on 'More' on the right-hand side.
A list of options will be displayed. On this list, click on 'Calculate Size'.