You cannot think of video resolution in terms of UIView dimensions (or even screen size, for that matter). The camera is going to record at a certain resolution depending on how you set up the AVCaptureSession. For instance, you can change the video quality by setting the session preset via:
[self.captureSession setSessionPreset:AVCaptureSessionPreset640x480]
(It is already set by default to the highest setting.)
Now, when you play the video back, you do have a bit of control over how it is presented. For instance, if you want to play it in a smaller view (who's layer is of type AVPlayerLayer), you can set the video gravity via:
AVCaptureVideoPreviewLayer *previewLayer = (AVCaptureVideoPreviewLayer*)self.previewView.layer;
[previewLayer setVideoGravity:AVLayerVideoGravityResizeAspect];
And depending on what you pass for the gravity parameter, you will get different results.
Hopefully this helps. Your question is a little unclear as it seems like you want the camera to only record a certain amount of it's input, but you'd have to put your hand over part of the lens to accomplish that ;)