you've got some good answers already. And the designs; you can place them in the center so you see them on the table, or you can choose as a border so on the part that hangs over the table, both is possible. but in the center you can see your work better.
I decide the size of a tablecloth by measuring the table and adding some 10 inches on all sides, whether it's round or rectangular. The embroidery should be seen when the plates are set. Best try that out on the table in question, or if you can't place it on yours or even on the floor and play around with your own plates, then decide where to place the embroidery.
Bernadette,that would depend on what designs you were planning on using....i did a family tree tablecloth in projects for in-laws.....if you could give us an idean, then we could help you from there...if you plean to do a center design, then fold your cloth in half and then in half again...from there you can put a saftey pin marking the very center of your cloth.....