Nikki Sixx -- The GIGANTIC Diamond Engagement Ring

The average size of a diamond engagement ring is reportedly 1.18 carats -- but Nikki Sixx just DESTROYED the average ... by dropping a SIX CARAT stone on his fiancee ... TMZ has learned.

Sixx popped the question to 27-year-old Courtney Bingham during a trip to St. Barts this weekend ... and sources close to the couple tell us it was one helluva proposal.

We're told the evening started with a candle-lit stroll on beach ... followed by dinner in a tent littered with rose petals ... and when the moment was right, Sixx whipped out the ring.

Of course, Courtney said yes ... because ya know, it's 6 CARATS ... and you pretty much have to. No word on when the two plan on tying the knot.