Just pick an aspect ratio that fits the original.
The beauty of AE is that there are no limitations other than the size rule of 30K by 30K.
Now the other rule is the squeeze ratio. Keep that 1:1 at all times.
So it’s normal to take a larger frame and scale down in your workflow.
There are however some plug-ins and processes that will task your system due to the constant scaling. In some cases, there are plug-ins that do not support anything over HD.
Not sure if thats the case of late.