In regards to GPH vs. Tank Size....
I've upgraded two different turtle tanks in the last year. The two enclosures we have are "twin set ups" - they're exactly the same and I have NO complaints. I am limited on how much water I can use due to over all weight and the water bills I have to pay - but I'm keeping the largest, cleanest turtle tanks I can and the turtles are happy, and I feel they have better homes than many other people's RES turtles.
I'm using 75 gallon tanks a little more than half full. My full water line is 40 gallons. I use Fluval 405 filters. They are conservatively rated at 340 gallons per hour. Evaporation happens, and the tanks often contain less than 40 gallons.
That means, that our filtration is at a minimum of 8.5 x filtration. These tanks do not have too much current. Far from being a whirlpool. I have found that you can position the out flow so that there is very little current.
The turtles LOVE these tanks. I get away with cleaning the filters about every two months. The tanks cycle flawlessly and water stays crystal clear.
I assure you, a Fluval 405 running on 40 gallons of water contained in a 75 gallon tank is not too much filtration - I'd recomend this set up to anybody.
EDIT - Just re-read and noticed we're talking about rated tank size, not GPH... so my 405's are rated for 100 gallon tanks, I'm using them on 40 gallons... so I'm using filters 2.5 times larger than the recomended tank size rating... So, of course Jozep is right on the money...
Yeah... 5 times would be a whirlpool. LOL. I think that the guy who said 5x must have been thinking GPH.
In regards to the original post - My advice is to upgrade when the turtle is about 3" to 4" in length, and make sure that you upgrade to a 4' tank that you expect to be able to house the turtle for life. Do not waste your money on any a tank that will just have to be upgraded again.
