Lorie, it's 3 1/2" deep by 14" wide? That should be an okay size although 3 1/2" isn't very deep. You'll notice most pond plant containers are no deeper than they are wide, and are often fairly shallow relative to their width.
You want a plant to fairly quickly fill up the container with roots, so don't plant it in a container that is excessively large compared to the tuber.
I recently came across a great website with detailed planting instructions for water lilies. Let me see if I can find it, and if so I'll post the link back here. Just remember to use clay soil and work in some fertilizer while you're at it; no soilless mix, or any component - peat, vermiculite or perilite - unless you want it floating on top of your pond ;0)