I think this is a thinnest Arduino currently available as it can fit in a AA battery enclosure. Read more and get the idea.

http://techcrunch.com/2016/04/19/aaduino/?ncid=rss&utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+Techcrunch+(TechCrunch)&utm_content=FaceBook&sr_share=facebook