In the 
Floor Plan, 
Table Plan, 
Table Chart or 
Guest Chart pane click on the 
Colours and fonts button at the top of the pane.
Note If the name of a guest can't be fitted into the space available in the Floor Plan or Table Plan pane, the font will automatically be scaled smaller for that name to try to make it fit.
In the 
Stationery pane select the appropriate text and click on the Font and Colour buttons at the bottom of the 
Design tab.