Choosing the correct frame size may make the difference between feeling comfortable or not when you ride. Choose too big and the bike may handle too hesitant and slow.
Choose too small and the bike may handle all too nervous and the tip of your shoes may hit the front wheel during sharp turns. You can probably come across plenty of opinions on the subject and some of them may even contradict each other.
We have found a video on frame size for road bikes and an article on frame size for mountain bikes which indicate what to take into consideration when choosing frame size.