Whatever the diametre of the base of the foliage on your pine tree is the area covered by the root system. The tree will be taking all nutrients and water from the soil in that area and there will be a thick carpet of dead pine needles to smother any other plants. If you walk in a pine forest you will see very little growing under pine trees other than bracken.
It is not a good place to try and grow anything, unless you stand containers around the trunk, however, most pot plants need light and sun and pines throw deep shade so it may be better to wait until the tree is removed before trying to use the space it is taking up..