Change Paragraph height

Yes, you can make it bigger using CSS. Target that HTML with CSS to make the height bigger. You would add custom CSS to your themes stylesheet to target the textarea for that field and increase it's size using the CSS height parameter.

You have to be more specific with how you style elements to insure they override any other styles in your theme stylesheet or the Gravity Forms styles.

For instance try adding this to your themes stylesheet:

body .gform_wrapper .gform_body .gform_fields .gfield textarea {width: 300px;}

This would give ALL textareas on ALL your forms a width of 300px. If you only want to target a specific form you would do this:

body #gform_wrapper_X .gform_body .gform_fields .gfield textarea {width: 300px;}

You would replace the X in #gform_wrapper_X with the number/id of the form you want to target.