Create model in sketchup (work in mm) and use CADspan plugin to generate stl file.
(Follow instructions on CADspan website)
(Settings: Highest Gap fill & resolution).
Check stl file in miniMagics. This software is also used by shapeways (I think). On the right pane it says "how to fix my part"
If your model is ok than a green sign is showed.
Scale the model in accutrans and export as Collada
To do so select "Save with options":
-file type Collada
-next to "OSF set to" type 0.0254
-press "OSF set to" button to scale the model
Than check your dimensions! (Accutrans uses meters)
and press save to save in collada format.
Edit: In the mean time I found out about the STL4SU plugin. I prefer that. It works a lot easier than the cad-span plugin!)