How do I measure a .stl file in solidworks?
You can use the Measure tool in SOLIDWORKS. You'll most likely have to select points to measure between since mesh models (STL) are terrible to work with in SOLIDWORKS.
STL files import in several ways in SOLIDWORKS. Your best option will most likely be to go to Options - System Options - Import - File Format - STL. Then set the Import As option to Solid Body. It would be a good idea to set the units as well if you know them.
Importing with the default setting of Graphics Body is next to useless.