How to decompose projection matrix?
Do you have any "input data" from which you constructed the matrix (like fov, near plane distance, etc.)? Or is the matrix itself the only data you have?

EDIT:
Ok, so if you want to know the size of the near plane while you have viewProj matrix available do this:

a) compute viewProj^-1

b) vertices of the near plane in clip-space have coordinates (in OpenGL):
(-1, -1, -1, 1)
(-1,  1, -1, 1)
( 1, -1, -1, 1)
( 1,  1, -1, 1)

and on the far plane:
(-1, -1, 1, 1)
(-1,  1, 1, 1)
( 1, -1, 1, 1)
( 1,  1, 1, 1)

It's a unit cube spanning (-1, -1, -1) and (1, 1, 1) coordinates with w=1

c) take each of those vertices, mutiply by viewProj^-1 and finally divide all resulting components by the w-component. That gives you the frustum's vertices in world space. From this you can easily find world space planes constraining the view frustum or the bounding box or whatever you like.

I think it's not possible to extract left/right/bottom/top/near/far from the matrix directly. But since you can compute the world space coordinates of frustum's corners (as shown above) you can easily find left/right/bottom/top/near/far.