Hey guy!
You are so sweet. I think she'll be thrilled no matter what! And if you think that Tiffany's would make her heart sing then go for it!
Okay I do have a Tiffany's ring and LOVE it! ... Love the service, the idea of it,
the box, (and the bragging rights, just a bit! hee hee)... I do. BUT I wanted to tell you what I'll tell my little brother next week (who is starting to investigate rings to propose soon) - don't get it into your head that ONLY a DESIGNER NAME = QUALITY when it comes to jewelry. When it comes to diamond rings, quality can be found everywhere from Tiffany's to the little shop down the street - the good news is, gold is gold, diamonds are diamonds, platinum is platinum. A Tiffany's ring WILL be great quality... but A "QUALITY" ring does NOT have to be from Tiffany's (or other design house). Know what I mean? Most of the ladies on this site have very high quality rings that were not bought at Tiffany's. Some are higher quality than some you'd pick up at Tiffany's.
Tiffany's has a range in colour/cut/clarity that they stay within which is a nice safety net - it's easy to buy there b/c whatever you leave with will be quality. The colour will be from D-J for example and no lower. The clarity won't go below VS2. The cut (in round anyway) will be excellent. But you can get that ANYWHERE... and the point is, you can get a much BIGGER one if you don't buy it from there.
I think you need to ask yourself: Does she appreciate QUALITY over quantity, or DESIGNER LABEL over quantity?
Like would the fact that she could say it's from Tiffany's mean more to her than having a full carat (assuming they were the same cut/clarity/colour)? Because at the end of the day no one but you two will know for sure what box it came in... but if you think size is an issue, buy it somewhere else. Guaranteed you could get her a 1ct for the same price as a .6 one from Tiffany's!

Just my thoughts!
I lost my original "non-Tiffany's" e-ring and ended up replacing it there... but I can tell you when my DH proposed with a .79 ring with a normal black box, I was OVER THE MOON. She will be too don't worry!!