How large a tank for adult red ear sliders?

[rosaceae]

So I have kept my sliders for years and now they are adults. They are of 20cm (7.875 inches) long and 13cm (5.12 inches) width, and another 18cm (7.09 inches) and 12cm (4.725 inches)... they have lived together for years and one of them is male and another is female (the larger one, of course). How big should my tank be, and where to find it? My local stores do not even have a large enough tank and now I'm using a plastic box (largest one I can find in my area) that is supposed to store toys... and the box is obviously too small for the sliders. My home is a pretty small and cramped one, and I don't even have a bath tub I can use as a tank or something.
Now I note I do have a quite large plastic box for toys, but then i want to know what is the right size for tortoise first?

 

[Yellow Turtle01]

RES get big! And if the ones a girl, expect more growth! At least a couple inches.
Is there bullying? The male will want to mate with your girly RES over and over, which is stressful and can cause illness
A pair can be good in 150+ gallons, with sight blockers, and plenty of hiding and basking space. Try stock tanks- http://www.tractorsupply.com/en/store/search/stock-tanks -is there anything like this near you? They can come is a range of hights and widths, so very flexible!
With such a big tank, you need a big filter, too. I had severe ammonia issues when my slider were together, and ofter a few months, I got the helpful hint that they'd be cleaner separated... so now, they each live on their own. This summer I'm planning on a large outdoor pond, because the situation now STINKS... for them... and literally

 

[Momof4]

I have the 150 gallon stock tank and I love it. I only have one and it works out great! I hope you can work something out.

 

[mike taylor]

I big out door pond is what I use .

 

[Millerlite]

Pond is idea. Stock tanks are good options. 300 gallon stock tank can house them for life.

Kyle

 

[rosaceae]

thanks for all your feedback. i will try to get one if possible.

 

[Randi]

I believe the standard is per inch of turtle, you want ten gallons of water at least. However, bigger is always better and more comfortable for the animal. If I had a full grown slider, I'd do 150 gallons. Two, 300 gallons. Also, more water can mean less waste building up as quickly. Look for a custom tank builder in your area.

Ponds are awesome, you could even build a green house and have one inside! The weather where I am is way too cold and snowy for a year round outdoor home. Good luck!