Click on the "Apple" menu, and select "System Preferences…"
Select "Displays" from the Hardware options.
(1) Be sure to select "Display" from the top menu options.
(2) FYI, you can disable screen "Mirroring" with this option.
800X600 is the default. Far too small.
(1) Select 1024×768 (70Hz, if given the option) to set the display to a more comfortable resolution.
If you have screen mirroring activated, this may distort the display on your main computer, but will look correct on the SmartBoard.
***PLEASE NOTE*** After making this change, you WILL have to re-calibrate your SmartBoard. This setting should be retained whenever you connect your projector, so you should not have to change this setting again.