In your software, create a cube with sides=1, centered about (0,0,0).
Export it as an Ascii STL and edit the file with Notepad or your favorite editor.
You should see a bunch of lines that look like this:
vertex -0.5 -0.5 0.5
vertex -0.5 0.5 0.5
vertex -0.5 -0.5 -0.5
If the values are 0.5 (one half unit) then your software is exporting the STL correctly. If they are NOT 0.5, then your software is changing the scale before it exports the STL.
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As has been said many times, the STL format is actually unit-less. It doesn't carry Meter, CM, MM, Inch nor ANY indication of units. When uploading to Shapeways, you are asked what unit that you want "0.5" to mean. If your software is doing some kind of conversion, then you'll need to adjust accordingly.
If your software just refuses to give you control, then I'd suggest downloading a copy of the free NetFabb Studio Basic.. it can easily enlarge/shrink your models so that you can achieve a successful Shapeways upload.