For a lot of pieces the file size of the stl's is 56 and more kb. You can't combine twice and upload them (there is a limit by 64kb).
Do you mean MB?
Then I have a problem with the number of triangles and have no idea how I can reduze it. Questions about questions...
Shapeways provide a tutorial here
but how much you can simplify the model by is not easy to tell. There was some discussion about this here
but it looks to be rather complicated and it is not easy to see how the reduction of triangles affects the puzzle.