How big should an oriental rug be under a large coffee table?

There's no one right answer here, especially not having the exact dimensions of the table itself.

Figure the important thing is to have at least 1'6" of carpeting extending out from underneath your table. More is perfectly fine, a little less would look somewhat awkward.

Another trick you may want to consider is laying down newspaper. Doing so, you can get a good idea for what different size rugs would look like. However, it's important to note that Oriental Rugs come in strict standard sizes, straying very little from the norm.