There doesn’t seem to be a way to place a ruler of any way to measure dimensions of an imported STL file. Am I missing something? This seems like it should be there.
STL format doesn’t contain unit information, meaning it’s size depends on what you choose when you import it. E.g. if you choose mm, a cube could be 10x10x10 mm, but if you choose let say, inches, then the same imported file will be 10x10x10 inches.
If you still want to measure it, at the moment the easiest way would be to draw a line next to it, The length of the line will be shown in the middle bottom part of the screen if you select it.