The configuration file is automatically created the first time you run DOSBox. The file can be found in:
(MAC OS X) “~/Library/Preferences/DOSBox 0.74 Preferences”
The file is divided into several sections. Each section starts with a [section name] line. The settings are the property=value lines where value can be altered to customize DOSBox.
And then change the line
to something larger. Might work…