Once you know what to look for, finding the right size tablecloth the first time is a breeze. Some of the process, like the length of the tablecloth, depends on your preference. Sizing a tablecloth for square and rectangle tables works the same way. First, measure the length, width and height of your table. 3) Find the tablecloth size that is closest to the numbers you just added. In this case, the closest size is 70 in x 124 in. Here’s a rectangular tablecloth on a rectangular table:.