Newbie Question - inserting a 1080 video into a 768 composition

Add the 1920-by-1080 (16:9) footage into a 1024-by-768 (4:3) Comp.  Scale the footage as needed (probably using Layer > Transform > Fit to Comp Heigh).

Then use that1024-by768 Comp as the layer with the mask in the current 1021-by768 Comp.

You'd refer to this as nesting (using a Comp in a Comp) or pre-Composing (moving footage into a Comp such that the resulting Comp replaces the footage in the current Comp).

Personally, I'd go and take a photo of the house from your projection position, then import that photo into the 768 pixel comp and draw your masks onto the house image.  

When only the projection surfaces of the house are visible, make sure the house image is above the projection content layer on the timeline, then switch the Track Matte option on the lower layer to "Alpha Matte".  This will use the masked house on the upper layer like a stencil, revealing the lower layer only where the house was visible.

You can now freely scale the lower layer to fit the house shape.

Don't use a mask, but a layer as track matte (can be a shape layer). This way, you seperate the mask from the footage. To fit your 1080 video, you just need to scale it. The mask stays as untouched "placeholder".

*Martin