The use of different sizes will affect the design and the message of the design. But you’ll find it really important to use different sizes to give emphasis to some parts of the design. For instance, in a newspaper, headlines are always bigger that the text for the news story. On the other hand, banner headlines which are used for the most important news story, are bigger than the other headlines in the paper. Same as in designing, you will find it helpful to use various font sizes.