Check the dimensions of your oven; cast iron pans are ideal for searing things on the stove and then moving them into the oven for roasting. It would be a shame to have a great pan that was too big to fit.

I'd lean toward a big one, 12-15". I wish I knew what size mine at home was. the reasoning is this: that's the right size to make one large round cornbread, based on the recipe on the cornmeal box.

I have 3 sizes. I use the large one most often - you can sear one piece of meat in it, or you can make a giant pot of grub for a bunch of people. It's not like anything really goes to waste by having too big a skillet (except, I guess,the greater BTUs needed to heat the larger skillet, but then hey, it continues to radiate heat a while and heats your house a little too, so you get the benefit of it in winter ;) ). Meanwhile, I use the tiny one on rare occasions when I just want to fry 1 egg or a small amount of diced potatoes. I find I use the medium-sized one almost never, because what for? You can do everything in either the biggie or the baby one. The middle size doesn't seem ideal for anything - except maybe camping or something.

The middle sized cast iron skillet was designed for the bonking on the head of marauders in western movies by old biddies.

I also have three - one tiny one that will hold like two eggs (makes the perfect omelette), one that's about 12 - 14 inches and a mongo 16 - 18 inch one. I use the 12 inch one the most - the problem with the bigger one is that it's not as good for sauces where you don't want it to spread out too thin and maybe burn. I use my cast iron pans for everything and I think the 12" is the most versatile.

I can never believe that people need to buy new cast iron pans. Are you sure there isn't an ancient one in the compartment under your stove? Don't they come with rentals? Don't they have them at yard sales for a dollar or in piles at Goodwill anymore?

A 12" skillet is pretty large. You can use it to cook lots of bacon, or braise a roast. But you can still lift it, and it should be okay on your burners. I just got a 12" cast iron skillet(at Goodwill), and it's quite nice, though a bit difficult to store. I use my 10" one the most. It was my Mom's, and the surface is scoured to a slick satin finish. If you are patient, you'll find nice old cast iron pans at Goodwill.

The medium sized one is perfect for 1 generous grilled cheese sandwich, as well as the afore-mentioned marauder-bonking.

You're mostly right, mgl--I've probably given a dozen of 'em away over the years, but it's not as easy to find 'em in thrift stores as it used to be (and I haven't seen any during my last couple visits). Like a lot of things, there are many more people looking nowadays. Honestly, some of the appeal of buying a new one comes from supporting the Lodge folks.

I have a 10" one because I can't lift the heavier ones without steadying it from the opposite side of the handle, which gets tricky when the pan is roasting hot.

In a similar vein, SO bought me a huge ass dutch oven that I've never used because I can't pick it up without putting a lot of body english on it. I hate to imagine trying to wrestle with it when it's heated up.

We also have a dinky frypan that's about 6" across and I never use it because it's just too damn small but I'm going to try theora55s grilled cheese sandwich idea soon.

Also, garage sale. Don't buy one new, they come with a waxy/shellacy coating which has to be scrubbed off before you use the pan and that's a giant PITA.

I vote for 10". 12" is just a bit too big for the stovetop and a bit too heavy. to handle comfortably.

Also, I've found that the coating on new ones will be baked off in the seasoning process. (Coat pan with high-temp oil, bake at 500F for an hour, voila, seasoned. Do not wash with soap!)

I have 2, a 12 inch Lodge that my dad bought for me when I moved and a 9 inch medium one which my grandmother passed down to me. I use the 12 inch a lot more even though the patina is better on the older one because it is better for cooking for 2 or more (I like leftovers).
12 inches can do 2 eggs and a side of potatoes at once.
Go big or go home.

Big. Big. Big.

Another vote for 12". With a helper grip opposite the handle. I have a 6", an couple of medium 8" to 10", a square grill pan, and the big 12". The behemoth sees the most use.