Box2d and sprites

Box2d and sprites
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i have really little practice with box2d, but basically it seems to me that it is possible
to attach to every sprite a box2d shape (that is, a box enclosing the sprite itself) and let box2d to check
for collisions.

Now, suppose that our animation is composed of N sprite (for example, a character walking) and that the sprites are not equal in terms of width and/or
height of their minimal bounding box.

How do you do with box2d? Compute an average box to enclose the sprite or change the box width/height at every sprite change?
The first method seems to fit well only those situations where the sprites bounding boxes are similar.

If that is not the case, the second way is surely the most precise, but i wonder if that is computationally heavy (i mean, to change
the size of a box everytime we change the animation frame).

Advices/ideas welcome!