what is the thinnest cable for microphone, single or multipair?

Lavaliere mic sized cable?
All the major audio cable manufacturers make lots of sizes. Canare, Mogami, Belden, West Penn, ect., ect., ect
I use the Canare L-4E5 Star-quad mini and L-2E5 mini for 'bag' interconnections. Not as thin as lavaliere cable, but thinner the normal.

mic sized cable, to wire a pair of mic (mkh 30/40) in windscreen on a boom.

I bought a 500-ft roll of Canare L-4E5C to make ten 50-foot "stereo mic" cables with 5-pin XLR connectors.
I REALLY LIKE that cable! It is flexible to lay flat on the floor. And it is very small, 0.189 inch / 4.8 mm diameter. And lightweight for packing as checked baggage on international flights. I hauled 10 "stereo" cables, and 10 "mono" cables on several international trips, and I love using them at home, also.

"A narrow profile version of L-4E6S. Specifically designed to save space and reduce weight during remote field expeditions or confining installations."

I have several stereo mics that use 5-pin XLR, and I also made several "Y-adapters" from a pair of regular female 3-pin to male 5-pin XLR, so I can use a single "stereo" mic cable for a discreet pair of co-incident mics (or M-S) And, of course I also made several adapters for the other direction as well (5-pin female to a pair of 3-pin male)

http://www.redco.com/Canare-L-4E5C.html