Schedule C line 30 requires square footage of home, but doesn't allow any input???How do I enter the square footage?

That field is populated from inputs on Form 8829. Click the grey quickzoom button next to the line for a quick link to enter 8829 information including square footage of home. For reference this is found by looking at the form in Forms Mode.

Please see the sample screenshot below for the screen where you'll enter the square footage information.