Set NSWindow Size programmatically

It is actually seems that +/- need to be reversed to keep window from moving on the screen:

NSRect frame = [window frame];
frame.origin.y += frame.size.height; // origin.y is top Y coordinate now
frame.origin.y -= theSizeYouWant.height; // new Y coordinate for the origin
frame.size = theSizeYouWant;

Use -setFrame:display:animate: for maximum control:

NSRect frame = [window frame];
frame.size = theSizeYouWant;
[window setFrame: frame display: YES animate: whetherYouWantAnimation];

Note that window coordinates are flipped from what you might be used to. The origin point of a rectangle is at its bottom left in Quartz/Cocoa on OS X. To ensure the origin point remains the same:

NSRect frame = [window frame];
frame.origin.y -= frame.size.height; // remove the old height
frame.origin.y += theSizeYouWant.height; // add the new height
frame.size = theSizeYouWant;
// continue as before

Swift version

var frame = self.view.window?.frame
frame?.size = NSSize(width: 400, height:200)
self.view.window?.setFrame(frame!, display: true)