Minimum window size of a Mac OS X application
Use -[NSWindow setMinSize:] to set the minimum size programmatically, but you can also set the minimum size inside Interface Builder (look at the tab with the sizes).
To make the application quit when the window is closed, you need to add this to your app delegate:
- (BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication *)theApplication{ return YES;}