Take this with you to the beach and you’ll certainly be the center of attention, mostly because a lot of people won’t know what the heck this huge polygon with numbers is.
No one’s stopping you from actually using it to play D&D, just make sure your friends are okay with it. Trust me, there are people who take their D&D seriously, and will not stand for inflatable d20s.
The inflatable d20 has a diameter of 17 inches; no word if it comes in other colors. You can order one at ThinkGeek for only $12 (USD).