I have a N551JX-DM192H. The SSD is Kingston smsm151s3128gd.
It is a Half size msata drive. Not very user accessible, and not very upgrade-able.
There are a few half size 256gb ssd for sale, they are more expessive, and not the fastest drives, and hard to find.
Ive ordered a dvd Hard drive caddy, going to move 1tb drive to that and add a 2.5 SSD, probably an 850 Evo.
Probably the best option.