Just FYI, I wouldn't say that a 1 carat is necessarily "too big" -- that largely depends on where you live, actually. Larger stones are more popular in the Northeast than elsewhere, and it is my understanding that in certain circles in the NYC area something smaller than 1 ct. may now be regarded as paltry. In most parts of the country, though, a 1 ct. would be considered a generous-sized stone, so downsizing from that by a quarter-carat or so would definitely not mean going tiny. Any woman with a solid understanding of what life costs isn't going to feel slighted when being gifted with a half-carat+ diamond.
I'd say that size is not really super-important to most brides in most places, as long as the ring is in proportion. With a solitaire, if the setting overbalances the stone, then it will look tiny no matter how large it is; getting the right setting is key. The style of the cut matters, too -- a shallower cut stone will look larger because it will be wider across the top.
Get your jeweler to let you play with the wax molds once you choose your stone. Good independent jewelers keep wax versions of many, many settings and can have that setting cast in whatever metal you want. Play around with them and try the stone in a bunch of them, since it is wax you can just do it with a tweezers and see the effect immediately. You are looking at a round cut so orientation doesn't really matter, but if you were looking at a shape with a direction, I'd also tell you not to get stuck on the conventional one to use. I have the emerald-cut stone in my ring set transverse, and it looks MUCH better that way than it would have in the usual vertical setting. A good jeweler can usually make minor changes in the style of the setting to give you what you want, whether it's shifting the orientation of a stone, shortening the height of the prongs a bit to give the ring a smoother outer surface, or changing the surface finish of the band (ours are brushed rather than polished.)
One other thing about putting your own ring together. Be leery of any jeweler who tells you that the setting you choose is too valuable re: the price of your stone. If it is YOUR ring, and you can choose to set a stone in any setting they have as long as it will fit securely. The intrinsic value of the metal is irrelevant, and people who harp on it are trying sneaky tactics to goad you into buying a larger stone.
Of course after you propose in case she is lurking!!!
Anyway, I hope you have a great vacation!
and think, well, I don't want to get my fiancee a "used" diamond, the deals you can find in pawn shops are pretty incredible if you know what you're looking for in a diamond, and you can have the stone pulled and reset in a new setting for very little. 
