A critical question when selecting a perfect engagement ring is that whether or not she will like it. A little research will go a long way to getting it right. Consider her lifestyle, ask her friends and relatives for any possible likes and dislikes regarding jewelry or try to discreetly find out from her on your own. Any effort expended in this regard will be worth it, even if you don't come up with much information. At least that way, you will have made a "good faith" effort.
The first aspect to consider is the metal which your ring will be made of. If you are not sure which metal to choose, consider platinum or 18K gold, the two most popular choices. Both are lustrous, durable and, most importantly, beautiful materials to work with.
If she has taken you window shopping for rings, she may have already shared this important information with you and your jeweler. If not, don’t sweat. There are several covert ways to get the size right. You might consider using one of her rings as a guide. Maybe she has a discreet friend or relative who might know her size.
If you are still concerned, it is alright to present the ring first and size later. In fact, rings today are stocked in the most common sizes anywhere between size 6 and 7. Most jewelers can have the ring sized in fast fashion. Because of the delicate nature of the setting, it is always best to have the ring sized once instead of twice due to a sizing mistake.
An alternative approach is to shop together. The universal tradition is to surprise your prospective bride with a marriage proposal, but if you feel it's in your best interest to get some direct input from your spouse-to-be, then you may opt to have an inexpensive, simple ring to present when you “pop the question” and tell her that the "real" engagement ring is something that you two can pick out together.
If your bride wants to wear her wedding band on the same finger as her engagement ring then you can opt for settings with matching bands. You can be as ornate or as plain as you wish. Take some time to look at the range of options that are available to you. Many couples find that picking out wedding bands together is a an experience they will treasure for a lifetime.