As far as I can see, a boule is a round ball of dough. Is there a difference between that a a miche?
I don't think there is a strict definition, but to call a boule a miche, I suppose it ought to be around 1.5 to 2.0 kilos, at least.
I concur with David's definition with one addition….. BIG Boule = Miche = BIG appetite *^ }
Wikipedia tells me that miches might have been created out of necessity when people in small French villages had to share a communal oven. They didn't have the luxury to bake daily so what they baked had to last until their next turn using the oven.
I'd been wondering since buying bread at the local patisserie. A miche was always round, so I started wondering if the boule might be the dough before baking. Lately I've been buying the pain belge. And I haven't been baking my own because I haven't had the time. Their miche levain is also very good.