Edit Text in Preview

Hi there

I'm wondering how to edit the text size in an editable preview document. I have a form that I can clearly input text in, but I can't seem to change the size of the text or bold or underline it.

I have clicked the markup tool and a tool bar appears that appears to have that and many other functions, but when I change them it does not change any of my text features.

Can anyone help?
Cheers