STL: Triangles, file size and 3d printing.
Let's break a few things down to basics.
A .stl file format is the standard file type accepted in the world of 3d printing. Yes, there are others, but more often than not a 3D file that you submit to a printer will be a .stl file.
A .stl file is made up a series of triangles used to define the shape or volume of an object. When you export a 3D model from a CAD program into a .stl format, the program turns your model into a series of triangles in order to define the shapes you have drawn. A square is no longer just a square, but two triangles that share a common edge. Round objects are no longer perfectly round, but become facetted. A sphere is no longer a perfectly curved surface, but instead looks like something Buckminster came up with.
It's a good idea to keep this in mind if you are either drawing a model to be 3d printed, or prepping an existing one. Basically, if you have a lot of curved elements on your model, the associated .stl file size will be bigger than if it were all squared edges.
The goal is to create the smallest number of triangles that can sufficiently describe your 3d object. Anything beyond that is extranious information your computer has to deal with a calculate. Excessive triangles can increase file size exponetially and can potentially create a world of headaches for you down the road.
How many triangles do I need? How many triangles is too many?
Tough to say. There are a lot of variables involved in answering that, but suprise, a lot of them go back to things you need to keep in mind if you plan on 3d printing your model anyway.
Tolerances: Know what will and won't resolve on the output machines you might be using. Adjust your meshes and triangle count accordingly. There is no point in having a finer mesh than what the machine can output. That just makes the file size bigger than it needs to be.
Scale: Like tolerances, know what your model will look like at its printed and final output scale. A tiny curved lip on something might not even show at your printed scale. If not, get rid of it or don't export it (put small details like that on different layers).
Unfortunately though, a lot of times it will be tough to control # of triangles and some of it will be out of your hands. If possible though, keep some of these ideas in mind when drawing, converting or dealing with .stl files. The less triangles, the smaller the file size, the easier it will be for yours (or other peoples) computers to deal with the files.