ISTM that you are most likely to trip on the edges of the carpet, so I would suggest sizing and placing it such that you would not encounter any edges as you walk around the pool table. (4 x 6 under the table or 9 x 12 (or larger) with the table centered)
JMHO

why do you want to put a rug underneath? Unless you get a rug big enough not to trip over when playing....which would have to be bigger than 9 by 12, because that only leaves 2 feet at each end of the table.
I think it would look very silly to have a rug just under the table...
Linda C

IMHO, in many cases a rug under the table looks nice.

Extending out 2' might be a little skimpy, especially if you have some 'big boys' playing.

The room is large and will have tile.
Do you have any concern for bouncing balls hitting the tile, and possibly damaging it? hmmm....

Here is a link that might be useful: Google images rug under pooltable.

Thanks for the suggestions....I am not really worried about the balls hitting the tile.....! I want the rug more for the look than anything else. But the price of a rug larger than 9 x 12 is more than I really want to spend. That is why I needed feedback....I agree that a small rug just under the table would look "wrong", especially in a large rm. Thanks everyone. If anyone has a links to rugs at great prices ......Please post them.

Thanks again...!