I'd start calling about a month out because sometimes it can take a while to hear back. And *always* call a few days before to reconfirm all the details of your order.
It depends on whether you get a 1-tier mini cake or a 2-tier minicake. It depends on how much cake you, your husband and your son like to eat. I am a sugar fiend and can still only put away about half of a 2-tier minicake in one sitting, cuz it's just so much frosting.
A 6-inch will definitely be too big, even if you just get a 2-layer buttercream one and not the ginormous 4-layer fancy kind with fondant over the buttercream.
This shot shows the scale of a 1-tier minicake:
It's harder to tell, but this one is the best I have to show the scale of a 2-tier mini cake.