To check the web space you have used on your 123-reg hosting package, please do the following:
Login to your 123-reg control panel.
Scroll down to the web hosting section.
Select the relevant domain name from the drop down box and click on the Manage button.
Click on the Web Space Checker option in the Website Help & Diagnostics section.
Here you will see your Web Space Usage Results.
You can also view the results for a particular directory by entering it in the field and pressing the Check Now button.