There are two basic measurements to deal with in 3D printing.
The first is a valid printable object.
Shapeways has identified the minimums for:
[list type=disc]
[*] Size
[*] Thickness
[*] Wall thickness (For hollowitems)
[/list]
They have validated through testing that anything less than a certain value (different for each Material)will not produce a valid print. I.e., it would be so thin it would be sure to break.
That being said, the actual print resolution is much much finer than these thresholds. In WSF it is down to .2mm (200 microns)
I am not privy to the actual algorythm they use to calculate a valid print, but rest assured, these are there to keep you, the customer, from ordering something they know you won't be happy with.
You can read about Materials here:
https://www.shapeways.com/about/material-options