One of the first questions I usually get asked from a gentleman when enquiring about a Bespoke Engagement Ring is 'what size should I get?'. Well, it's always a little bit of a shocker for them to receive the news that the leap between carat sizes is substantially smaller than the leap between the price of each carat !
Personally, I mean if you can afford it then a 3 carat is a nice size (she says, ever hopeful hers will grow) BUT i've made many beautiful engagement rings that are bang on one carat.
If you are looking for a bigger look without the price tag, you can create a small diamond halo surrounding the central diamond to make it appear bigger or you can purely respect the stunning colour and clarity of the stone by keeping it intentionally simple.
So really the answer lies not in the size of stone but in the design. What are you hoping to achieve? What is the look you are going for? I also find that by having a different cut of diamond, say a princess or a marquise cut then you can often find they "appear" larger in size once set.
Whatever you decide, she's a lucky lady in the first place and anyway if she's so preoccupied by the size of the stone, then maybe it's a great opportunity to adjust the waving red flag in front of you and set off into the sunset.
Just a thought.... lol