First of all, open up your Finder:
Usually, your computer log-in name will appear on the left-hand side of the window, if it doesn't, press the following key shortcut:
Command + Shift + H
The Command keys are located on either side of your spacebar and look like this:
Right-click (secondary-click) on your log-in name and choose 'Get Info'.
A new window will open and you will see your Home Directory size (it might take a few seconds to calculate).
Please fill that number into the Google form.
When in doubt, ask a Coach!