Supreme Leader Snoke
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.