The easiest way to measure a pool table is to measure the inside "nose" of a cushion to it's other side. Since some companies will use different factors in rail thickness and different size cushions it easier to measure this way. Below you will see where the nose is located on this 8' pool table.
Typical sizes are:
7′ — 40″ x 80″
8′ — 44″ x 88″
Pro 8′ — 46″ x 92″
9′ — 50″ x 100″