Most tables are 30 inches in height. When chosing a round tablecloth size, the following are the drops for the different sizes:
A 90" tablecloth drops to the floor on a 48" table and half way down on a 60"
A 108" tablecloth drops 3/4 of the way to the floor on a 60" table
A 120" tablecloth drops all of the way to the floor to the floor on a 60" table.
NOTE* It is not recomended to use a tablecloth that drops all of the way to the floor.
I recomend the 108" tableclothes on 60' tables. It allows for your guest's feet to go under the table without pulling on the tablecloth and upsetting your centerpieces.