Three weeks minimum in primary then bottle or keg. No need for secondary unless dry hopping, adding fruit, oak chips, etc. Be sure to take a gravity reading to make sure it's really finished.
If you do secondary (I mean actual secondary fermentation where you add fermentables to the brew to feed the yeast or if you're dry hopping) then the 6 gal bottle will be fine, since more CO2 will be produced. I'd use a blow-off tube, though, just in case things get really active. Otherwise, there's no need to use a secondary. You'll still get clear beer and you won't get off-flavors from sitting on the yeast cake.