It will take any 2.5" M.2 2280 SATA SSD (but not NVMe).  It's not limited to 256 G -- that's simply the largest one Dell supplied.  
I purchase Dell Vostro 15 3568 in 2017. Now i want to upgrade the storage device i.e from HDD to SSD. But i am confused which SSD is better. I choose Samsung 860 EVO 1Tb but on dell owner manual it says that M.2 SSD 256 GB, 128 GB specification. So i am confused that it only support SSD upto 256GB or it can be upgraded upto 1TB. I just want to clear the doubt.
Want to know which SSD is suitable for Dell Vostro 15 3568