First, determine your budget. One general rule of thumb when buying a diamond engagement ring is "two months salary." This is just a guideline, it's not carved in stone and your first consideration should be what you can comfortably afford.
Deciding on carat size is really about striking a balance between size and quality. If she prefers larger jewelry items, and you are working within a budget, you can still find a larger diamond of excellent quality by selecting one which is graded slightly lower in terms of color and clarity.
Remember that slender fingers make small diamonds look bigger. If she has small fingers, a 1-carat diamond will look proportionately large -- and an even larger stone may appear stunningly big!
Think about what sort of setting will hold the diamond. You'll have to be sure that the setting you choose is made to fit the carat weight of your diamond.
Learn about the 4 Cs of Diamonds before you begin shopping.