how do i change the home office size or square feet for self employent?

The fastest way to do this in TurboTax Desktop is to do it in Forms mode. If you are not already in Forms mode, click the Forms icon in the blue icon bar. Then next to your name at the bottom, click on “Forms” and select the T2125 that you want to edit. Then scroll down to Part 7 – calculating business use of home expenses, and you’ll see the details of your home office size there. 

To return to EasyStep Mode, click the EasyStep icon in the blue icon bar.