Circles come out smaller because a circle is printed as a polygon inscribed inside the circle among other things. Please read nophead's blog: [
hydraraptor.blogspot.com].
As far as the wall thickness of a single-walled piece, it depends on your layer thickness and the width to thickness ratio. There's nothing particular about 0.7 mm. For example, if you're layer thickness is 0.3 and your width over thickness ratio is 1.5, then the wall should be 0.45.