I say go for the bling! Let me put it this way: when I was first shopping for sims, I was targeting 1.25-1.5ct size because I thought that was most believable for us, given our cars, home, personalities, etc. Then I came across a steal of a deal on a 2ct OMC and that's when I got into antique cuts. I realized that there are places where consumers can buy large diamonds for the same prices as you'd pay retail for smaller stones: ebay, estate sales, pawn shops, auctions, private party, etc. Is it risky? Sure!
But here's my point: if anyone mentions the size or cost, you can always say something like, "Between you and me, we never could have bought a diamond this big, but then we had an opportunity to buy a diamond from [insert source: estate sale, auction, etc.] and it was such a steal, we couldn't pass it up! It cost less than most 1 carats!"
Alternately, you can say you inherited the diamond from a relative, or that you saved for it for a number of years, etc. There are countless valid "reasons" that someone could wear a diamond larger than expected.
Take me, for instance! My position in life hasn't changed, but I now wear a 9mm diamond, and could easily wear a 5ct if I wanted to, and no one would question it because that's just how I roll, lol, big diamonds