The first step is to measure your table. Round tables should measure the diameter, while oval tables measure to the longest point for the length and the width, and then rectangular tables just use the length and width. If your table has an extension that you often use, but you only want one tablecloth, make sure you measure with the leaf in so that you have the correct length for your party settings.
Next, you need to determine how much drop you'd prefer on any given side. For more formal tables you might want a 14" drop while a casual look may be just fine with an 8" drop. If you're unsure of the best drop size, consider putting a big piece of fabric over your table, like a flat sheet or blanket, and determine the length you'd prefer.
Take the drop size you've determined that you like and double it, add that number to the length and the width, and select the tablecloth size that most closely matches your desired measurements.
Example: Table Measurements 46"W x 84" L, desired drop is 12", 12*2=24. 46+24=70" x 84+24=108". Select Tablecloth 70"W x 108" L.