Size changes after upload

I got a model of a half of pliers, and I sized it down to a few CM in blender. When I upload it it's under 1 CM. Is that a Shapeways error or is there something I did?

Most likely something you did. Some general hints:
- if the size is off by a factor of 2.54 you have a confusion between metric and imperial units somewhere.
- some software packages use different units for modeling and for export.
- the base unit of the VRML format is one meter, STL however is in arbitrary units so you will be asked to choose one on upload. The other
formats contain an entry for the unit system used.
- if you cannot figure out where things went wrong, get netfabb studio basic and use its "scale" function from the "part" menu to adjust the scale.

I scaled it to hundreds of milimeters across with netfabb, and of course it said it was too big. So I scaled it to 20 milimeters across, and it says it's 2. I have no idea why it's doing that.

Check if there is some very small part sitting some distance away from your model - something small enough to be automatically
removed by shapeways' software, yet messing up your scaling in blender and netfabb.

I'm having trouble moving around in Netfabb, I've never used it before. I checked in Blender and I don't see anything.

Pick the object in Netfabb, it will turn green. you will also see the bounding box around the model. Does all of the model fit in this bounding box? Or as @mkroeker states 'is some very small part sitting some distance away from your model' making the bounding box bigger.
If the bounding box is the model only then the size shown will be correct. Do not use 'hundreds of millimetres' the size is in mm. So if the model now is 2 use 'Part - scale' type in 10 to make the model 20mm. The number you type in is a ratio of the original size.
Once you have scaled it you will have to export the model with the new size. 'Part - Export part - as Stl'