checking ssd size

After purchasing some SSDs at a 50% discount on Amazon, I wanted to check if they are really 256 GB SSDs. 

https://superuser.com/questions/470949/how-do-i-create-a-1gb-random-file-in-linux

With the aim of filling up at least 100 GB of space on the disk, I just tried

truncate --size 1G foo

shred --iterations 1 foo

That completed in less than a minute. But the same thing with 100G instead of 1G - truncate completed instantly, but shred was going to take time. After ten minutes or so, I cancelled it. Then tried copying over the 100 GB file from one disk to another etc, that was also not complaining, so I concluded that the disk was fine.