Hi, I've used sketchup quite a few times too, everything @bluelinegecko is saying sounds like good advice, I just wanted to throw in my workflow for reference, because I've had similar scaling issues.
Here's what I do:
Model everything scaled up (I like to work in meters and pretend they're millimeters)
When finished I save the file, then I open up the group or component of the object and use the measuring tape tool to take a measurement of a know distance, i.e. 3 meters, then before I do anything else I type in 3mm, sketchup asks if I want to rescale the active group, I say yes, it scales it.
Next I exit the group and move it to the origin point, most times I then explode the group down to bare lines.
Finally I export to .dae and upload to Shapeways
After all of that I just undo back to before I scaled the group and resave, so I can come back later and work on the model at the larger scale if needed, but my export model is at the correct size.
The process seems tedious but you get it down pretty fast. Works like a charm for me, haven't had any issues since I started doing it this way. I avoid the actual scale tool like the plague, if I use it to scale then I always get size errors.
Hope that helps!
-Noesis