This is one of the best BMW wheel database sites I've found; it's in German, but pretty intuitive to navigate (tips below):

Der BMW-Treff Felgen-Katalog
"The BMW-Meeting Wheels Catalog"

First, it's easier if you click "Alle zeigen" in the left pane to load all of the wheels instead of having multiple pages.

Second, they use commas in place of decimal points, e.g. "8,5" rather than "8.5".

Finally, it helps to know these words & phrases:
  • "Styling-Übersicht" = "style index"
  • "Baureihe" = "series"
  • "Felge" = "wheel"
  • "Bereifung" = "tires"
  • "Auswahl zurücksetzen" = "clear selection"
  • "Alle" = "all"
  • "ausnahme" = "exception"
  • "außer" = "except"
  • "Größe" = "size"
  • "Einsatz" = "used"
  • "vorne" = "front"
  • "hinten" = "rear"
  • "Allrad" = "four-wheel"
  • "Speiche" = "spoke"
  • "Doppelspeiche" = "double-spoke"
  • "Sternspeiche" = "star-spoke"
  • "Kreuzspeiche" = "cross-spoke"
  • "Verbundrad" = "composite wheel"
  • "Radialspeiche" = "radial-spoke"