They could have just fit some really tall guy in it and got away with it but I begged and pleaded and they were nice enough to build a Darth Vader suit that actually fit me.
“The droid we seek is aboard the Millenium Falcon in the hands of your father… Han Solo.”
A version of this scene was shot with Kylo Ren mask-less, and the real masked version digitally replaced it later. This behind-the-scenes footage comes from the Star Wars: The Force Awakens VFX Breakdown - Before and After (edit: I found a working link!).