Well......we have three. Two right now are in food use - one large upright (size of a fridge) and one 7cuft. The 30cuft is right now use for....other stuff.
Right now the 7cuft is full, the upright is about empty - I'm actually thinking about removing the rest of the stuff and putting some in the small chest, and some in the huge one, I'd have to partition it (not a big deal).
When hunting season rolls around, we will need every inch of the upright and a big portion of the big chest freezer. Apparently there is going to be a new deer season early this year, and rather than being able to "max" out at 5 deer each (DH and I), we will be able to get an additional 5 each early. Considering I usually get about 80-100lbs of meat off a deer when I process it - well, that is a whole lotta venison!! We also will have wild turkey, goose, duck, and grouse in there.
We use the small chest for everyday stuff. I do not like using the freezer above the fridge because it's just horrible for food quality for more than a day or so. I use it when I am planning meals ahead- pull them from one freezer and put in the fridge freezer to have handy. I also use it for freezing stuff on trays before putting in bags to go in the big freezers.
If you're not used to having freezer space (that's good!) then a 5 or 7cuft would be best. It has a small area - I have ours in the laundry room - it's smaller than the washing machine. Home Depot and all those stores sell them. If you decide to go the whole stockpiling or buying whole pigs or half cows regularly, then you might want a bigger one, but Craigslist is usually really good for those. Heck, I see small chest freezers there for $50 often.
Also, get a NON frost free freezer. Long term food storage is not good in frost free freezers, due to the food constantly being partially thawed on the outside and refrozen repeatedly. Think how your ice cream looks if you eat some of the carton then leave it for a while in the freezer - gummy at the top, and plain yucky. Doesn't happen in a regular non-frost-free freezer. As for defrosting - if you keep it mostly full, and keep the lid shut most of the time - don't stand there daily bathing in the cool air - then it will take a very long time to build frost. I do find uprights build frost faster though. I've never defrosted my chest freezers unless I'm moving - my little one hasn't been defrosted in 3 years now, and I just looked - it has about 1" of frost on two sides, the other two are clear.
I LOVE big freezers, and I think every household should have one!