You can adjust the branch width for all branches in your map via the "Options" dialog.
To open the "Options" dialog:
- Switch to the "Tools" ribbon
- Click "Options"
- Switch to the "Display" tab
- You can use the "Force title to display on one line" and "Max width" controls to determine how long your branches will be.
(Note that it is not possible to change the width of individual branches separately.)