...come in standard sizes. You'll find a 70 inch round but probably not a square. Some brands (usually more formal styles) have a 70 x 84 oblong...you could cut and hem it (or just leave it! It would only be an extra 7 inches on two sides.)

Two 72 inch runners laid criss-cross on your table would be pretty, too...assuming the table top doesn't REQUIRE complete covering like mine does! I've also seen runners going paralell down the table.

If you're creative, buy a pretty fabric and make one. A 52 inch center with contrasting bands on the perimeter would be striking and you'd always know you had it on straight! If you find a fabric (probably in home decor) that is at least 70 inches, you could use pinking shears to cut it and not even hem!

Good luck!
Lenora