If it's a park ski, most people who ski park could ride a 176. Most would prefer either 171 or 181, but a lot of people wouldn't mind a 176. So if you're only going to choose one size I'd say somewhere around 176.
If it's a powder ski, 181 or 186 would probably be the most common, although a lot of people would like a 191, I think you'd get more people that would be able to ride a 181 or 186.