It's not really the volume that matters as much as what your grain bed depth ends up being. And even here, there is some flexibility. If it is too deep, you will have good efficiency but will risk slow runoff or a stuck sparge. If it is too shallow, you will have an extremely smooth runoff but possibly poor efficiency since it will be easier for it to channel. This can be especially problematic if your manifold design leaves large dead spots.
If you are batch sparging, however, the latter is not an issue - only if you fly sparge.