This question has been asked a number of times and it has been given different answers to it.
This all depends on the map and configurations of the map and that tells of the configurations one should use.Exactly what configurations do you use?
There's about two things which need to be altered based on map.
(Note: I use the mouse to play instead of keyboard so my settings and advice might be off for keyboard users.)
The mousebox needs to be adjusted based on the game play and movements that one is likely to make.
If one is making movements which require lots of speed and are mostly dodging based, one should use a small mousebox.
This allows one to make very fast direction changes and permit one to dodge well in most situations.
The case for using a large mousebox are on maps like Quickshots and the Jumping skills.
Here it is mostly about being accurate with the movements instead of being very fast.
This is to move quickly and make sure one is accurate on the shots or the jump.
This requires a good amount of precision which makes it difficult to master.
(Note: on maps where I use a large mousebox I tend to have a small chatbox.)
The middle size mousebox is a compromise between the areas of small and large.
This is where being very skilled and precise with the movements of the mouse is needed.
This can be used on maps like Apocalypse and generally leads to good success.
Generally the speed and the amount of precision determines the size of the mousebox one should use.
Larger mousebox = slow and more precision
Smaller mousebox = fast and less precision
Some random thoughts and advice of mine.