Either build your canvas for 1080p and scale your capture source to that when building the scene, or build your canvas for your capture resolution and click the "scale to" button in the output pane to reduce to 1080p when recording. You can do it either way, see whichever works best for you.
I tend to build all my scene collections with the same canvas resolution as the game I'm playing has, but my understanding is that this takes more resources than building the canvas for the output resolution and then just resizing the sources to fit the scene.