The size of a rainbow in units like centimeters, inches, or miles, is not a meaningful question. If you think for a moment, you've probably see rainbows in the spray from a garden hose and you've seen rainbows that appear miles away. They are different sizes. It does make sense to talk about rainbows in terms of angular size. See
Rainbow Size.
EDITED: Warning, reading the article associated with the link will: (1) Tell you more than you probably ever wanted to know about rainbows, and (2) Possibly take a considerable part of the magic away.