Measure the table first, then choose the cover. A 54 x 108 inch rectangular cover is the standard drop for a six or eight foot folding table, while an 84 inch round sits on a 60 inch round table with about a twelve inch drop at the sides.
The drop is what makes a table look dressed. Add roughly 12 inches on each side of a rectangular table and 12 inches all round on a circular one; less than that and the cover looks stretched, more and guests catch it with their knees.
If your tables change from event to event, a cut-to-size roll is usually cheaper per table and you trim it to the exact length. Pre-cut cloths win when the same table has to be set the same way every week.
Two practical checks before you buy: the film weight (heavier film survives a buffet run without tearing) and the pack count, because a 20-pack covers ten tables if you double up for a long event and only five if you change covers between sittings.
Buy on Amazon
Covers that match the sizes above, with live prices and pack counts on Amazon: