The following is my own learned experience from the previous 20+ years. There is no guarantee that I'm right, or that I communicated clearly. But if it gives you some ideas for your own cast iron lifestyle, then I'm happy to have helped!
The next biggest is 7 inches across. It's good for 2-3 egg omelettes, for cooking medium sizes of meat or fish, that sort of thing. Next is our largest skillet at 9 inches. It's the least used, because it's heavier, so more work to move around and clean. But if you have guests or family over, it'll be there to handle the cooking of large portions.
Finally we have a 10 inch flat cast iron griddle. This is probably tied with the tiny one for the most use--you can make scrambled eggs on it, or grilled cheese sandwiches. Fry onions, and it'll handle modest amounts of ground beef if your skillets are busy with other ingredients.
Don't overwhelm yourself. Buy one piece, make all your mistakes, get accustomed to it. I'd suggest the flat griddle (I like the round ones, because that's a better match for the burner underneath).
Here's why you don't buy pre-seasoned: they don't prepare the surface to a fine enough texture.
Go out and buy some 60, 120, and 220 sheets of sandpaper. They come in packs of 5 or 10, and that'll be enough for several skillets. Break out that coarse 60-grit, and sand all of the inside surface (i.e., the parts which will touch your food) until it looks and feels uniformly smooth. The grit and the iron it takes off will accumulate, so periodically wipe it out with a damp paper towel. Don't use your dish towels, because this stuff is messy.
Also--don't rush it. Be thorough.
Wipe it out again, then go back around with 120 and do the same thorough job. It'll feel noticeably smoother after this phase. Finally come back around with 220. When you're done, it should feel almost glass smooth.
I should mention classic American cast iron, versus the China stuff you'll probably buy. There's this thing called grain structure which bounds how finely you can finish the metal surface. Chinese cast iron is "good enough", but if someday you live with old American cast iron, you'll be shocked at how much finer the metal is.
Finish up, and wash out the cast iron with soap and warm water. I know, they told you to never put soap on your cast iron! That's true, but only once it's seasoned and in service. Give it a final hot rinse, cleaning it out with paper towels and getting it all the way dry, inside and out.
Seasoning in all the instructional videos has you running your oven, timers, all that modern stuff. I bring cast iron into service in the winter, because we heat with wood and a wood stove is the perfect way to get your cast iron seasoned in. So if you have an oven, use somebody else's instructions, or try to get the same effect that I'm describing here.
To season the surface, you wipe it with something which then cooks onto the metal surfaces. What to wipe down with? I've used vegetable seasoning, and I've used lard. They both work. For the last several years, it's been lard. (We have a local supplier, so we know it doesn't have any surprise ingredients.)
Get your wood stove up to its room-heating temperate (my chimney thermometer says around 600 degrees Fahrenheit). Wipe down the cast iron on all its surfaces--handle, underside, all of it. I use paper towels, but if you want to dedicate a cloth to it, that's fine too.
Then just put it on top of your wood stove. An hour or more later when you think of it, take it off (hot mit!) and wipe it down again. It's a little tricky wiping on another layer, but you'll get the hang of letting the towel (or cloth) be in contact, bunched up enough to keep your fingers away from the hot surface. Then back on the stove. Repeat when convenient.
That's it! Do this a few days in a row, with 3 or more rounds per day, and the lard or seasoning will be baking into a nice non-stick layer.
If wiping on a lard layer, do it with a paper towel when the iron is warm but not yet hot. It'll spread nicely, and then you can use the water trick to tell when it comes up the rest of the way to temperature.
Fatty foods don't need you to add butter/oil first. For something like a hamburger patty, just make sure the meat sizzles when placed onto the pre-heated cast iron.
If something does stick, use a spatula and slide it between the food and the cast iron surface. You can usually break (most of) it free. With teflon pans, you'd cringe at doing such a thing! You might scratch its precious surface, and off it goes to the landfill. With cast iron, the worst case is you season on a new layer or two, and life goes on.
If at all possible, clean your cast iron right after use, while it's still hot. You never put it wet into the drying rack. After cleaning and drying, the residual heat will make sure any last bits of moisture evaporate away.
Often, especially with maturely seasoned cast iron, there will be nothing sticking to it afterward. Wipe it clean with a paper towel, then wipe it down with your lard or seasoning (this protects from rust). Clean it while it's still up at cooking temperature, and you're done. The residue of that same heat which killed the bacteria in your cooked food will now take care of tiny hiding remnant hoping to live on on your cookware.
Sometimes you'll end up with a pan with lots of stuff cooked onto its surfaces. This is where the newbies run back to their teflon junkware. But this simple and beautiful insight comes from my friend, the late great Dan Schumacher: fill it with boiling water to cover the crusties, then put it back on the stove and bring the water back to a boil, running your spatula across all the crusted-on mess as it starts to boil. It will all let go, and your cast iron surfaces will emerge victoriously from underneath the grasping food residues.
You'll learn to detect residue while wiping with a paper towel; the paper will rasp across it. You can not season on top of crusty residue. Don't try. Please. Deal with it.
A copper scouring pad is your friend. Like the paper towel above, you can lightly run it across the surface of your skillet, and feel where it catches on something which shouldn't be there. Then run it across that spot with a little more pressure, and add a bit more until it breaks or scrapes free. As I said before, you're not going to destroy the skillet. Worst case, do another round of seasoning. Copper seems soft enough to me that it doesn't do much--if any--harm.
Let me know how it went!