How big should the gfs engagement ring be?

Niche79

Hey guys, So ive been in a holding pattern from buying a 370z because of this issue. I wana pay cash only for her ring but at the same time get her something very nice. We have dated 5 years now, and I have known her about 8 years since she was in college at UNF. Now shes a 2nd year resident doctor in GA with only one more year to go starting in June. So how much is ok without going overboard on a ring. Most of her Doctor friends only have half ct. diamonds but I think that is too small for a doctor. Basically I told her I wouldnt buy a 370z till I had her ring. SO what do you suggest, Ive searched rings and have figured I would buy it from blue nile online as I have known others who purchased through them and their prices are much cheaper than in stores. I tossed out the 2ct number but she said she would never wear it to work so now im thinking between 1 and 1 1/2ct. I dont wana give it to her till our alaskan cruise in july so I have time to decide and save up for more cash, or just keep it and save for a nice downpayment on a Z haha. Thoughts and tips of course are welcome. Tell me what you bought etc?

Thanks!