How can I change the font and the font size in the chat?

You can change the font size this way:

1. Open a chat window with any user
2. Click on the wrench button, that is just above the typing area
   
3. On the popup window that will open, click the “Font” tab
   

On this tab you can select the font and the size of the letters.
Use the “B”, “I” and “U” buttons to set bold, italic or underline styles for the text.
Use the “F” button to select the font, font size and several other font related options.