They're sold as 1TB for that price. But if you actually write that much, you'll find that only part of your data is there. The rest is garbled or zero.
There's tools to restore this (on windows/linux), and it'll show up as a smaller size when you run them. You can also use such tools to set any fake size you like
And if you send in your dna three times, you can find all three if your ancestries! Really tho, there was some funny news on this a while back when identical twins took those tests and got different results