A .dae file contains scene data when uploaded to SW. So if you are setting unit measurement under scene data in Blender. when you upload to SW, unit measurement is set.
A STL contains no scene data, depends on the Blender units in the dimension box in N panel, you select what measurement you want a blender unit to equal at upload.
As for file formats that carry color info that SW recognizes,as was stated above, a blender unit is regarded as meter. So if you created a model with the dimensions in blender units of 5 x 4 x 6, and you want it to be printed in cm, scale by .01. Apply scale. Select meter at upload.Why this works is after getting scaled, dimensions are .05x .04 x.06 in blender units. when you select METER at upload, it gets printed at .05m x .04 m x .06m. Which in mm is 50 x 40 x 60. or in cm 5 x 4 x 6
If you ever scale a model in Object mode, ALWAYS apply scale(CTRL A). And remember for file formats that do not contain scene data,you have to set at upload what you want a blender unit to equal.
Keith
You can set unit measurements in scenes. If you choose METRIC, the scale factor beneath it can change unit measurement to cm(.01) or mm (.001).
For Imperial( inches), set scale at .0254. If you decide a primitive,select NONE, add it, will be in correct proportions. Then reselect either Metric/Imperial