Change the size of a Rich Text Field


I have no real knowledge of CSS/Javascript etc but I have a form that I want doctors to complete on the screen. Each box relates to different sections e.g.

Medical History



I have put these in as Rich Text fields on the Database side of things. But now I would like these boxes to be bigger and fill up the screen more. At the moment they just appear on the left hand side and only 1/3rd in to the middle of the screen. I'd like them to have the width of the screen or close.

How do I go about doing this?