I thought it would be helpful to try and provide you with a more comprehensive answer, so here goes.

Each of the file formats that you might choose to use (for example, OBJ and STL) is actually storing the object that you have created as a table listing all of its vertices as individual points (X, Y, Z). These are connected to one another to create an assortment of polygons which form the actual shape you are working with (the shape you see after your conversion to STL).

The catch is that some software allows the option of using mathematics to create the illusion of a smoothed surface when the object you are using is displayed on the screen of your monitor (which is the shape you see in your original OBJ file). This is not the 'real' object, just a smoothed interpretation of it. Since what we are printing is the 'real' object, you need to turn off the smoothing effect to see what your design will really look like when printed (how to do this depends on what software you are using at the time). You may need to add more polygons, for example, to create an object that is actually smoother, rather than just looking smoother.

Now to the issue of scale. Since each of the points in your object is stored as an (X, Y, Z) co-ordinate, the numbers have an associated unit, which is different for each type of software you might be using. (This is similar to metric vs. imperial measurements, in that one unit in software may not equal, say, one unit in the real world). To make your object the right size, you need to find out how to convert the units your object is currently in (as an STL) to a real-world measurement in centimetres. Once you calculate the conversion factor, you must then scale the entire object in software so that it is the correct number of STL units to equal the real-world size. (Example - I want my object to be 20cm wide, but my software uses 1 inch units. I know that 1 inch = 2.5 cm, so I make my object 8 inches wide. It is now a 20cm wide object when printed).

To find out exactly how to do all this, consult the documentation for your software (for smoothing and file format specifics) and the online documentation for Shapeways (to find out how to convert to centimetres, how big your object can be, and other design limitations). I hope this helps to answer your questions.