Reusable element library for AE?

I'm creating some animations with simple characters that have elements I'd like to reuse. How do you set up that kind of library? For Instance: Eyes & mouths. Do you recompose the elements? is that the best way - if you want to be able to animate the element, let's say, an eye blink - but you want to have an untouched / clean version that is totally reusable?

And what size do you make the elements? Do you make them the to thesize of the main composition> A little eye - in a 1920 x 1080 composition? I come from an illiustrator background. So I'm a little confused about how to begin to set this up. Can anyone suggest any learning resources? Many thanks - and apologies - maybe I'm being too vague about my goal? Many thanks!