extending the Header section post text area in BlogsMu

I have enabled the Header section under 'Section' in the theme options and entered Header Section Titles and post text.

These show up fine on the home page but the 'Post Text' for this header section is limited to 2 lines of text. The default text when the 'Post Texti is left blank has three lines.

I can't work out what to adjust.

Can you tell me where/how I would allow more text to be shown in this area?

Thanks