Engagement ring

Yes. Do you always criticize gifts given to you? Also, it doesn't matter what you've "always told" your fiancé, as this is a person and capable of making mistakes or just telling you, no. Or perhaps they just know more about jewelry than you and understand carat is only one aspect of diamond quality.

What exactly is the problem with the carat size? Is it too large and causing issues when wearing it?

I agree with CeCe. Is it too big or too small? If it is too big it seems reasonable to go smaller without hurting your fiancé’s feelings. If it is too small, then that would be pretty hurtful to say to your fiance.

Changing a stone often requires changing the setting as well just FYI.

Assuming the stone is smaller than what you wanted, you should be prepared to pay the difference for the bigger stone. Diamonds are expensive. You wedding is still 2 years away, you have plenty of time to save if that's what you care about. The difference in cost could be tens of thousands of dollars depending on what your expectations are... did you talk openly about cost when you talked about what you wanted?

You know FH best as far as how he will react.