Does it matter wether the coördinates of your vertexes are small or big?
EG Is it better to zoom out 100.0f and draw a cube with a radius of 10.0f of zoom out 1.0f and draw a cube with radius 0.1f
Does that influence the precision of the depth buffer?

The floating point numbers represent numbers with a constant relative error, (fixed point >> constant absolute error)… I think it must be the same.
However it can depend on how the hw handles values…
The precision that you can get with zbuffer depends by the ratio of the far and near planes.