Are hit boxes tweaked using an editor?
I think that collision boxes can be calculated in most of the cases automatically without much of trouble. No need for user intervention.
Cheers!
Depends on the game. But yes, often they are tweaked in some way. Maybe not necessarily an editor, some just get inflated/deflated by some small percentage. For example, most bullet hell shooters, the hit box is smaller than the sprite. Or for 3rd person action games, a cylinder or sphere is used for collision, and mostly tweaked to feel right.
This is another one of those questions where the answer varies from game to game based on the specific needs of the project and the tools used.
For some projects, and using some tools, hit boxes can be automatically calculated. In some cases this is good enough, while in other cases it might not be easily possible to automatically calculate a hit box or manual tweaking might be needed to provide better results.
In some editors, you are provided a choice of different options which are automatically calculated for you: you might choose between a bounding circle, bounding rectangle, or "pixel perfect" options. In some editors you can manually add or adjust your own hit box/geometry. Some editors provide both options so that you can use whichever best meets your needs.
As mentioned in above, a designer will sometimes want to make adjustments beyond simply finding the geometry which best fits the graphical representation so that they can tweak game difficulty by using a smaller or larger hit-box than might be automatically generated; you might use a slightly larger hit box for GUI elements to make them easier to interact with, and (as above) you might use a slightly smaller hit box for the ship in a bullet hell shooter to make the game slightly more forgiving.
Hope that helps! 
There was a game dev video floating around with the Overgrowth guy talking about how he set up his character, animation and movement. And one of the early parts is how he chose a sphere, and the size of it.
Ah, here it is: http://www.gamasutra.com/view/news/216973/Video_An_indie_approach_to_procedural_animation.php
It's a fun video, even if most of it is probably well beyond what you are asking for.
