Say I want a particular 10" sub. And say the recommended sealed sub enclosure, published by the manufacturer, is .4 - .6 cuft. And say I have a nice custom made enclosure that's .9 cuft. What will happen? Is it that big of a deal?
You'll be fine. I generally build my sealed boxes a bigger than recommended for the added low bass extension. Alot of manufacturers use ultra small boxes as an attractive selling point to people that lack the room for a large box, and IMO they go overboard with their ratings in many cases.
What will happen if it is WAY too big, in your case around 1.5-2.0 cubes, is that the subwoofer will not have sufficient back pressure and will bottom out or flap uncontrollably. Another advantage of a larger than recommended sealed box is that the driver will reach full excursion with less power but your overall power handling will be decreased a bit as a result. A trick that some have used to fill a vastly oversized box is to screw some 2x4's in the corners and I have even seen people throw a few big rocks in there. This obviously isn't recommended for a vented enclosure. It is worth pointing out that adding any filler material such as dacron will actually make the box theoretically larger.