Best to keep them larger than you need. Perhaps even create your PSD file at 1600X 800 if your AE project is 800 X 400. That way you can always scale up your layers to double size if you want to.
In short, Yes, as long as the layer names are the same they will come as you adjusted them. One tip, for importing PSD’s into AE, if you add layers, it will not recognize the new ones when you reload your AE project, one precautionary pre-emptive step is to add dummy empty layers Name them whatever you want) If you need to add an element later, just drop it into one of those layers and it will be imported. You could of course also simply load that single changed layer on it’s own and drop it into your comp, but it’s nice to save a few steps.
Nicolai