I am trying to insert a 1080 video into a 768 composition. This is for projection mapping onto my home. My projector is 768 native resolution, so I do not think I can change the composition. I have a mask that sections a part of my home to put the video onto. I thought that I would simply apply mask (copy-paste from template) to video and scale. However, that scales my mask and now it no longer fits the form of my home. Thank you for your assistance.
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".
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.