At the top of every page, there is the 'Create' menu item, within that there is 'Upload' - browse for your file, select the units of measurement and upload - it is as easy as that
Shapeways does not change the size of your model, however you must make sure that you have the correct units of measurement and that your 3D models files are not only compatible with Shapeways expectations, but that the model structure itself also complies with the design guildelines for the material you want your model printed in.
There are plenty of tutorials to help you along your way, all accessable via the 'Create' menu at the top of every page.