Actually, given how scrupulously clean Harry keeps his summoning circle, particles of dirt probably do break it. Just saying.
And don't worry, I don't have the photographic memory stunt either, I just finished rereading the series last week.
I think it's dependent on the degree of power of the circle, and its nature...at most. The circle to contain the loup garou was an integrated combination of circles meant to inhibit both spiritual AND physical passage and also any combination of the two. I'm fairly certain that simple dust wouldn't have broken the circle if it took so much to render the original useless, and only Kim's mistakes in the circle's design and control let him get free and kill her. So, dust is out on that.
Harry's newest circle and what he used it for probably required a little more cleanliness than standard circles, yes. Even so, since it's literally physically impossible to get rid of all dust in the air or on the ground without an enforced clean room environment (actually, even in such an environment there's still dust, it's just less pervasive)...dust very likely doesn't break any circle, nor most bugs, since many are too small to be seen and are prevalent in even the cleanest homes. Also, gases don't break them, which are material objects. So either size matters, or something about the establishment of a circle automatically clears the area directly above and around the line. It would also mean any person in a circle for too long would asphyxiate. Not very reasonable at all, even for magic. Harry's just being excessively careful, which doesn't really hurt I supposed. Better safe than sorry.
As for how high a circle goes...probably to a height sufficient to top off whatever's inside, or to the first thing that could've broken it otherwise that intersects it from above, whichever is lower. Don't really have any clue because it hasn't even been mentioned in the least in any book or short story. But, that would be pretty reasonable I'd think.