I have 2 4-5" Red Ear Sliders, 3 1.5" False Maps, and 1 1" Stinkpot. They are incredible pets, but are complicated and expensive to get set up properly.
The other pet forum I belong to is a big turtle site with many, many knowledgeable and patient owners who will be more than happy to help you learn how to properly care for your turtle. There are multiple forums, and I've been a member of two or three and the one I'm a member of is the best, in my opinion.
Check out Austinsturtlepage.com, which has a lot of good turtle care information, along with a link to the forum.
As someone was so kind to point out, turtles will NOT stop growing simply because you keep them in a small tank. A possible explanation for what really occurs is that the turtle lives in an unhealthy, stressful environment leading to a premature death at a small size. Red Ear Sliders, which are a very common turtle, can live to be 25-30 years old in captivity, if kept properly. Keep a turtle in a small tank, or lagoon, is like keeping a cat in the closet for it's entire life. Hardly humane, right?
The general rule of thumb for tank sizes is 10 gallons per inch of the turtle, plus 10%. So, a 10 gallon tank would hold a hatchling turtle for an incredibly short amount of time. In fact, our 1" stinkpot is in a 35g tank (though the water isn't filled that high because they can tire easily an drown). If you add a 2nd turtle, you use that same figure but cut it in half. So, two 1" turtles would need, at a minimum, a 15g tank or so. And that would only last for a couple of months.
The best approach, if one can afford it, is to get the biggest tank possible instead of purchasing multiple tanks as the turtle grows. Check your local craigslist or ebay for local pickup listings. You can often find a 55g tank and stand for under $100 bucks. And, when you buy filters, lights, etc, always buy online...even at petsmart.com. The prices in the stores are incredibly higher. For example, a great canister filter is the Rena Filstar XP3. It's almost $200 at petsmart, but at petsmart.com (and other sites), it's around $100. You can save a lot of money this way.
Look at it this way, you probably spend a few hundred dollars on your cat for food, litter, toys, treats, etc over the course of a year. With a turtle, you spend all of that at once. It's a tougher purchase, but if you get a large tank (for example, a 100g tank), then you're set for a few years rather than a couple of months. Then, all you have is food and maintenance. A thing of food will last you for a long time if you are a feeding them properly. You should supplement their diet with aquatic plants (can be found at petsmart), along with suggested vegetables. I actually feed my turtles tortoise food because it has a higher vegetable/fruit content and a lower protein content, which means that, in theory, I can feed them more...but I don't.
I think that, as pet owners, we want what is best for our pets, within reason. Obvously, we can not provide cats with a large natural environment, nor can we provide turtles with a river or lake...but it is our responsibility to provide them with something as close to their natural habitat as possible. Hatchling turtles are born swimming in water multiple feet deep, yet some inexperienced owners keep them in 2" of water.
Turtle ownership can be overwhelming. I typed a novel and barely tapped into the depths of it. But, it's a very, very rewarding experience...something totally different than owner a cat or dog.
If you have ANY questions, check out that austinsturtlepage.com or send me a PM and I will be glad to answer your questions.