If you are using AE's camera tracker you can either resize and do all your production work in the resized comp or do all of your production work with the original footage and resize upon rendering. The ratio doesn't matter for Camera Tracking, but you can't Camera track footage of one size, then resize the footage and use the same camera track.
Pillar boxing or letter boxing is not a problem and it is necessary if you want to maintain the same aspect ratio in your final footage because media players and screens have standards. Without knowing a lot more about your production process and your intended delivery format it's hard to give you any better advice.
The plates will need to be 3D tracked very accurately
Then why do you even use AE? You know, it has no way of doing anything with your recorded data and can't be calibrated, so this makes no sense.
Hi Rick thanks for your reply, basically I'm shooting some 4k back plates for visual effects but the client also wants the the plates supplied at 1080p.. so ideally not letterboxed, but I just want to check this won't skew any of the data. The plates will actually be tracked in PF Track but I just wanted to pick the brains of the community here as many will have downsized 4k for various projects.
As mentioned to Rick the plates will actually be tracked in PF Track but I just wanted to pick the brains of the community here about downsizing 4k for 3D tracking.