I absolutely had to have this option, and while expensive ($475 vs. free on many cars), it is 100% worth it for me.
The opening is about 36" x 17", and appears the same as the one without it.
2013 F10 535i Carbon Black M-Sport
2011 F10 535i Dark Graphite (Sophisto) Metallic (traded)
2009 Nissan GT-R (sold)
BMW CCA # 493835