its not the size of the ring, is the quality of the diamonds on it and the metal used.

so seriously, every women wants a pricey ring, and its not mostly because of you, its coz women like to show off with their friends, women when in company of other women are really vane, as we like to show off our cars or even our pets, women show off their clothes and jewelery, i think if you can (if you can afford with no major sacrifices) you should get something pricey, but if you can not, better save for the furniture, electronics, a car, honeymoon, a house...... etc.
Honestly i dont even suggest you to get married, but if you have decided doing so well good luck.