The first time you run Burp, it is worth taking a moment to check your display settings. Burp lets you select different sized fonts for different parts of the UI, and you may want to change these settings, depending on your screen resolution.
First, look at the text shown in Burp's menus, tab captions, buttons and other text.
If you want to change the main UI font size, go to the User options tab, then go to the Display sub-tab, and edit the font size in the User Interface section.
Then restart Burp and check whether the new font is suitable.
Second, go to the Repeater tab and type some text in to the request panel.
If you want to change the font size for HTTP messages, go to the User options tab, then go to the Display sub-tab, and edit the font size in the HTTP Message Display section.
Then go back to the Repeater tab and check whether the new font is suitable (there is no need to restart).