I buy it for my Laptop, so it must be 2.5" SSD.
Anyway, I searched Google, and someone had the problem like me:
My experience: if you plan to buy a new Laptop which has large screen > 15", you should make sure that it has 2 slots for hard drive. In general, there will be 1 empty slot, so you want to buy a SSD to put it in like me. I suggest you must contact the manufacture of your Laptop to buy the Hard Drive Caddy.
I plan to buy ASUS Laptop, so here is the place you can buy accessories from ASUS: