How to save a rhino document as stl. and for what

In rhino selected ur model and click export selection ...
select .stl and then save the file
it tells you the parameters of the object that you select.
Automatically it trianguilate the model ... it converts each plane into triangles.
choose binari beacuse its smaller (1kb)

Another way to do i t.. ( if your model its to complicated its better to convert to malles than export it as a stl. ( select melle) you can choose how many of them you want. you can preview to see if thats what you want.
the recomendation its starting with middle melles, so its not that heavy your file..