Trouble with sizing in Fusion 360
Welcome to SE.3DP! 

First off, F360 isn't the best with STL files. If you're having trouble with constraints and dimensions, I would suggest watching this Maker's Muse video first: How to use Constraints! CAD for Newbies with Fusion 360.

Second, Fusion360 is very tricky with importing STL's. My steps below should help.


In the lower right-hand corner, at the rightmost end of the timeline, you'll see a little gear. When you click on the gear, click the very top option: "Do not capture design history". This puts you into Direct Modelling mode.
In the top left-hand corner, where it says "Model", and select "Mesh" from the menu.
Along the toolbar, in the "Create" section, click "Insert mesh". When that's done, go back to the top left where it now says "Mesh" and use the menu to go back to "Model".
Now that you're back in "Model", go to the "Modify" menu. In there, find the "Mesh" section, and in that box, click "Mesh to BRep". That will convert your STL into a Fusion360 file that you can edit.


Now, if you want to use constraints, I would suggest sketching out your object entirely in Fusion, making constraints and dimensions along the way. I know it's annoying, but it'll be easier to modify it in Fusion.
Hope that helps!