If you do use the scene panel to set unit scale, it does work exported as a .dae in blender uploaded to Shapeways which is wunnerful.
. I am also curious are you placing the file in a folder, zipping the folder, uploading, or zipping the file, and uploading it. Big difference.
As was explained previously SW gives you two options for conversion factors for .obj and stl., mm or inches.
If you have a 12 x 5 10 model, and want it printed in cm just scale it by 10, result is cm 12 x 5 x 10, or mm 120 x 50 x 100.
Determining the scale factor is good knowledge. The Scale factor is basically the dimension you want divided by the same dimension of the model.
So you modeled a ring with a inner diameter of 16 units. Someone wants to buy the model, but her ring size inner diameter is 20 units. You just divide 20 by 16, scale model by that amount.
Your other question, if your models length was 12, regard it as 12 mm, scale the model by 10. When its printed with the default mm, length will print as 120mm or 12cm with the other dimensions scaled same amount.