Xinha: Small width and height settings don't behave as expected

The width is not honored when it is smaller than a certain width.
When the height is too small, an extra scrollbar is added.

To reproduce the screenshot:

• Go to document type configuration
• Edit a document type with a xinha field
• Set the xinha field width to 300 and the height to 100
• Commit the changes
• Open a document of that type
• Click the Xinha field
  Expected: field is of the configured dimensions
  Acutual: see screenshot.