Frame size is the single most important factor in how your road bike will feel and perform. Start with the manufacturer's size chart, which matches your height and inseam to a recommended size—most road bikes range from about 44cm to 64cm. But don't stop there, because sizing varies significantly between brands, so a 54cm in one may fit quite differently than a 54cm in another.
If you're between sizes, consider your riding style and proportions. A smaller frame gives you quicker handling and more standover clearance, while a larger frame offers more stability and a longer, more relaxed position. When in doubt, it's generally safer to size down—you can make a smaller frame bigger with a longer stem or seatpost, but you can't shrink a frame that's too large.
For the best results, look beyond the nominal size and check stack and reach measurements, which more precisely define your actual riding position on the bike.