[bingaman_]

They come in three sizes, A, B and C. Since you're 6'4" you need size C. You can tell the size of the chair you're sitting in by feeling under the top part. If you're in the wrong size chair, you're probably not going to like it. FYI.

[mechanical_fish]

Yep. If it doesn't fit you, it doesn't matter how much it costs. I'm only 6'2" and the Aeron C is a hair short for my legs, so I can well imagine that many 6'4" wouldn't like it.