You cannot increase the font size in Brief Accounting. One option that can be used is to zoom in on the Brief Accounting screen - this is done by enabling "Zoom" in OS X Preferences:
Choose the Apple menu > System Preferences
Click Universal Access
"Seeing" tab > Set Zoom to "On" > Click "Options" button
Set as below > Click "Done"
When Brief Accounting is opened in the foreground:
To Zoom In:
To Zoom Out: