About Screen Size adjustment
If the used source aspect ratio is different than the projector aspect ratio, e.g. source is 16:9 and projector is 16:10, then black bars will be projected. In the example case a black bar on top and bottom of the image will be projected. With screen size these black bars can be eliminated. This makes it also more easy when using warp as the image corner will be taken in stead of the corner of the black bar.

