In The Force Awakens, Kylo Ren and General Hux go talk to someone who is shown as the hologram of a big, weird, and scary guy. They call him “Supreme Leader,” and he seems to be the power behind the ruler.
Who is he? Where does he come from? How was he tied into all of this?
Snoke follows the Dark Side of the Force. He is Kylo Ren's master.
He persuaded Ben Solo to turn to the Dark Side. Ben Solo then took on the name Kylo Ren.
His Wookieepedia page says:
Snoke was a male individual who served as the Supreme Leader of the First Order approximately thirty years after the Battle of Endor.
Kylo Ren and General Hux answer to him.
Snoke was a powerful figure, skilled with the dark side of the Force, who had trained at least one Force-sensitive, Kylo Ren, as an apprentice.
Currently, there's not a whole lot we know about him. His backstory will likely be expanded in the upcoming films.
As to why he appears so large, he is actually communicating to them via hologram. Since the projected holograms come in different sizes, it's hard to tell his actual size. It can be assumed he would probably be "normal sized". For more on his height, see How tall is Supreme Leader Snoke?
Snoke's design is similar to Star Wars concept artist Ralph McQuarrie’s early design for Emperor Palpatine.
That is Snoke. He is the new grandmaster: Kylo Ren’s mentor and Supreme Leader ("emperor") of the First Order.
There are very few theories about Snoke’s past.
He might be Darth Plagueis, Palpatine’s master, and have survived from his overthrow by Palpatine, since Plagueis could change the midi-chlorians into forms of life.
Snoke could also be Palpatine. Somehow Palpatine could have survived when he was picked up and thrown by Vader into the shaft. (That could be why Snoke is deformed and scarred.) He comes back and changes his identity. He appears big in holograms just like he does for Vader. (Empire Strikes Back)
