You can change the font size using Waveform Preferences Dialog Box. This dialog box allows you to set display properties of the Accelerated Waveform Viewer.
Go to Tools | Preferences | Environment | Appearances.
Under the Window list, select Accelerated Waveform.
Under Subcategory, you can change the font settings for the Waveform's Signal Grid Pane, View Pane, and Cursor View Pane.