If you are considering taking up cycling, one of the first things you should consider is the size of your bike. A well-fitted bike will provide years of use, but an ill-fitted ride will leave you in pain and prone to injury. Determine whether your road bike frame fits you by standing over the top tube with your feet on the ground. There should be just an inch or two between the top tube and your groin area. Any less and you might feel cramped, while too much space means may make it too hard to reach the ground.
Conversely best way to judge seat height is to sit on the bike. Once on the seat, you should be able to straighten your leg out on the pedals when they are at their lowest point.
Of course, all bikes differ from one another, and what fits you on one brand or model may not prove universal. To find the best fit, buy from a reputable shop where you can get professional help determining the best fit for you.