Depends on the cable, but it's not unheard of to be a foot or more in diameter. Here's a deep-sea fiber splitter:

http://i.imgur.com/qh4Gf.jpg

This is incorrect. A subsea fiber optic cable is about an inch in diameter. The splice cases and amplifiers are bigger.

It's not incorrect, the original comment is vague. I think it is more reasonable to assume they are talking about the entirety of the cable rather than just the carrier embedded within it.

Indeed I was. Thanks for the clarification.