The volume will need to be bigger than the LUN if you intend to take snapshots, see my answer to your other post.
With regard to housing multiple LUNs in a single volume; it depends on if the snapshot requirements for each LUN are the same, if not then they will need to be in separate volumes. Make sure and place each LUN in it's own qtree, however there are more things you may want to take into account. I suggest having a read through the Best Practices in the Tech Library: http://www.netapp.com/us/library/