I have made a few models- but I have no idea how to make it a certain size. I have made things with the intention that they will be pendants or earrings...but when I upload them, they come out too big or too small. Does anyone know how to control the size in Zbrush? Also- how do you make sure the preview picture doesn't suck when you upload it?
Use the free plugin 3DPrint Exporter for ZBrush, which is installed by default in ZBrush 4R2, available in the Plugin palette.
You will be able to define the exporting size of your model, in mm or inches, and then export it as STL or VRML.