I think you might be an
[Title:] Abstract, Mysterious & Chewable!
Not sure about "Mysterious", but (1) the notion of "orbits" is found e.g. in the pure-mathematical field of group actions, related to abstract algebra and (2) Orbit is a brand of chewing gum.
To find me you need look no further than behind the eye,
The orbit is an anatomical term for the eye socket.
Take your time to ponder and you'll see me in the sky,
Planets have orbits around the sun (and stars around the centre of the galaxy).
I can be shorter than a breath or longer than a year,
Orbital periods can be very long or very short. I don't know of any astronomical objects with orbits "shorter than a breath", though, and the submicroscopic things that would give much quicker orbits are better understood in quantum-mechanical terms for which the notion of "orbit" isn't quite the right thing.
I repeat myself, repeat myself, repeat myself I fear,
Orbits are periodic.
So large I am at times that I'm best observed from space,
If e.g. you want to identify exoplanets by observing their periodic effects on the stars they orbit, you do best to use something like the Hubble Space Telescope.
Yet so small I can be that I'm right before your face!
Perhaps this is about eye sockets again. Or maybe artificial things like gyroscopes?