I wouldn't do it. He didn't spend the $500-1000 for a ring with better specs because he chose not to and I believe if he's purchasing it, he has final say. Although it's nice when your fiance seeks your input, I feel like it loses all its meaning if you dictate how much more he should spend or how much bigger the diamond should be. When you think about it....the guy is spending a huge chunk of money on a ring AND in most cases, contributing to the wedding budget so it's not small change.
My ring is under 1/2 carat. I'm betting it's probably one of the smallest ones on here but I LOVE it. It's the one my fiance loved and knew I would love, within his budget, that is. You're planning to be with this guy for the rest of your life, right? There's plenty of time to upgrade for your 5 year, 10 year, etc, etc, anniversaries. That is the beauty of getting married, you can GROW together and hey...so can your ring.