I don't know the size but it takes a 10mm wrench to unbolt them which should mean that it's a 6mm bolt.
If you want a sample, pull off the third one that holds down the center console. It is behind the finish panel all the way to the right (about an inch to the right behind the hole for your missing bolt. There you'll find a bolt that is the exact same size. The only difference is that the exposed bolts are blackish and the hidden one is goldish in color.
