Tool to measure in STL files
I suggest blender. It's not the simplest of tools but it is free and learning it will improve your 3d printing skills. :-) (I write this answer also for future viewers of this question so I start basic).

Check here for another answer:
https://blender.stackexchange.com/questions/19772/how-do-i-measure-a-distance-between-two-points


Import your STL-file.
Press the Home key to view everything.
Select the model by clicking on it with your left mouse button. (Blender changed to left-click-select as of version 2.80)
Hit tab to enter edit-mode.
Press N (or use View | Properties) until the Properties panel shows up.
Select the "Length" checkbox in the "Edge Info" section of the Properties panel (see image below).
Select "Edge Select" mode (see image below)
Select the edge to measure by clicking on it with your right mouse button.






If you need to measure the distance between to vertices with no edge. Create the edge by selecting them and pressing F.
If you need to measure the distance between a vertex and any other point, select it and press E to extrude.