General settings are a part of the Preferences Window. The preferences window can be opened by selecting "Preferences" in the "Wirecast" menu (or Command-,).
Open last document on startup
When checked, the last document you used will be opened when Wirecast starts up.
default: on.
Show checkerboard background in Shot Editor
When checked, a checkerboard background is shown in the Shot Editor.
default: off.
Aspect Ratio
Wirecast allows you to device the aspect ratio of your broadcast canvas.
This is the relative width/height of the Preview and Live Broadcast area in Wirecast.
The aspect ratio should match the aspect ratio of the Encoder Preset you are going to stream with so that no squeezing is done by Wirecast to match your encoder presets aspect ratio.
As of Wirecast 3.1 when you selection this option, Wirecast will automatically reset any media that has "Respect Aspect Ratio" enabled.