Two very useful plugins were recently released by Pixologic. Decimation Master can reduce the polygon count in your model to manageable proportions (remember we have a 500,000 polygon limit per model here at Shapeways). 3DPrint Exporter allows you to correctly size and export your model to STL or VRML, both of which formats are accepted by Shapeways.
Both plugins are free and come fully documented. At the moment of writing, they are Windows-only. A Mac version should be coming soon, according to the website.
i am having problems getting my file to size correctly. for example I used the 3d print exporter plugin to change the size to 100mm in zbrush but when i go to upload it in shapeways it ends up being 10011 cm? can somebody help me?