I want to make a .stl file to be 3d printed through blender, but I don't know how I would see the real size. Does anyone have any solutions? Also, if you turn on smooth shading, will it actually be smooth IRL or just the shading?
Blender uses meters as its default unit scale. If you add in a cube, it will give you the option to change its size in the popup window. Set the cube to the desired scale and match the scale of your object to it or set the cube to 1 m and use it to determine the scale of your grid lines.
As for smooth shading, that just turns on normal averaging on the vertices. It does not add any detail to the geometry. The smoothness is an optical illusion from the even transition of light across the face. If you plan on printing, be sure to turn smooth shading off so WYSIWYG.