Woman here. This is totally a matter of preference and taste. When we were thinking about rings, the things I cared about were 1) I didn’t want a starter ring- I wanted to have the same ring for the rest of my life, 2) I wanted to like the style of the ring, because I was planning on wearing it every day for like 70 years, and 3) I wanted a center stone that looked proportional to my hands, and I have kind of big hands. I was open on what kind of stone I wanted for a center stone.
I would say that as long as you can afford it, this is a VERY rough estimate of what carat weights work- it is then up to the two of you also about quality, cut and type of stone.
1/4 carat or less is pretty small and petite. Can look great on the right hand and in the right setting, can also look small on some.
Around a 1/2- 3/4 carat is going to look pretty good on most hands, and it is just preference.
3/4 to 1 carat can be too much for some fingers, looks great on others but is eye catching.
Over a carat is pretty gaudy IMO.
I think my ring and wedding band together were around $5k. I have been wearing them every day for almost 15 years and still love them.
Good luck and congrats. You are going to have to talk to her, though!!