IMO the size of the ring should be a non-issue. If you're in love an onion ring should be good enough. Also I believe that basically if you cant afford a giant rock then a girl shouldn't require you to go all out and blow your entire bank account getting them a big ring, if they do then they're shallow and not worth your time....however if you are MORE than capable of getting a good sized ring and you get them the cheapest one available, it might say that you are moderately to very cheap. In addition to this, I believe that every girl has their own dream of their 'perfect' everything, the 'perfect' man, date, engagement, ring, dress, ceremony, honeymoon, etc...however 90% of the time they don't get those things...but will they still be happy in spite of it? Absolutely!! ....as long as you're not a total butt hole and at least TRY to make it as perfect as they want it.
But dont take my word for it..its just what I believe..and if I'm wrong, someone please tell me..