Comp sizes vary on what kind of footage you use. However, we urge you to use comp sizes compliant with the final delivery format. If you shot in 6k, but are delivering in 1080p, we recommend creating a 1080p AE comp and keep all nests, precomps and tracking comps in the same format. This removes issues when downsizing and headache with scaling tracking data (which often goes less than perfect). Also, scaling down during a render greatly increases rendertime, where scaling up during a render greatly lowers output quality.
If for some reason you really need to work in higher resolution comps, you can convert those higher res comps to the final output and insert them into the final AE file using a free script called Recursive Scale Comp which can be found here: Recursive Scale Comp. Note that you will need to isolate the scene first, possibly in a seperate AE file (duplicate).