Personally I found the automatically resizing chat box at the bottom of the screen ideal in FFXI for saving screen real estate during combat. Please consider implementing something similar for FFXIV.

Suggestions for chat/message box:
1. Auto-hide the tabs at the top
2. Auto-hide the input field and divider at the bottom
3. Auto-size the height to a maximum number of lines
4. Auto-size back to a minimum number of lines over time