A simple technique , depending on the printed size you want, is to go to Scene where you can set units, None, Metric, or Imperial. The scale factor is under the unit choice, if Metric is chosen, .01 scale will change a unit to cm. If .001 with change it to mm. If you chose Imperial, setting scale to .0254 will make a unit an inch. With Metric chosen and the model scaled to the size you want, Apply Ctrl A(apply scale, then you can input the value into solidify mod(
1mm thickness for example.). Interesting enough if you chose inches as the other option, you can still type in
1mm as the value, which will give the wall 1mm thickness.(or any other thickness you want.
You can see the values in the N panel in inches, or cm for dimension value.
What makes this simple, select all of your mesh, export as a dae(Collada). When you upload your mesh, SW detects the scene data. So your model will be printed in the dimensions of your bounding box values in the N panel. A .dae is one of the formats SW recognizes scene data on upload.
To double check the thickness, you can turn on edge length in the N panel, and select an edge. Extra good stuff if you have the Measure panel addon, it can display volume in object mode, plus warn you if the model is nonmanifold. For all of the steps, remember if a model is scaled in object mode, use Apply scale(Ctrl A) after it done.
An STL format is totally different..Chris Lowe has a good tut on it.
Keith