Personal space refers to how far away people stand or sit from each other, whether or not people touch each other and even what personal items are ours, and which are shared. The actual rules vary with culture, age, gender and how well people know each other. The whole concept gets very complicated by the fact that we learn most of our social rules from our families, but by nature, families follow different rules with each other than friends or strangers do.
There are two simple ways to gauge the rules on personal space. If your child is talking to someone, pay attention to what the other person is doing. Most people can't help but back away when someone comes too close. If your child gets this signal often, from multiple people, try to get him or her to increase the distance.
The second way is merely to observe. Kids who struggle with social skills need parents who act like anthropologists. Pay attention to what other kids at the school or playground are doing. If your child is too close or too far, he's sending a message he might not mean to.