Do it by eye, basically. Zoom out the 3D view in Blender so the cylinder is roughly the size you think you'll see it at the most and adjust the number of verts until it looks right. Do bear in mind that smooth shading hides a LOT, so you'll be surprised how, in certain circumstances, you can get away with very, very low vert counts. I made my partner a necklace a while back. The number of verts that make up the extruded shape for the cord on it? 3.