I am building a 6'6" casting rod. I want to put the new Minima two piece reel seat on it. I see their sizes range from 9mm to to 15.5mm. Which size is the popular for these type reel seats?

It's not about what is popular, it's what will fit the blank in the location you want to place the reel.  Measure the blank and order the appropriate size.   If it's in between sizes,  orderived the smaller and ream to fit your location.

Since structure is so important with skeleton seats which have very little area of contact with the blank, be especially careful to properly prepare BOTH the blank and the seat components before gluing.  You want to scrub both with an abrasive pad.  Easy to do and will maximize the performance of the epoxy.  Also make sure you get full epoxy contact with both surfaces.  The best performance will come not from deep grooves from very coarse sandpaper, but from a "deglossing" of the surfaces and removing of mold release from the seat parts.  Do not follow this treatment with any other action other than wiping the dust away.

To do this to the seat parts it is easy if you have a shotgun patch tip for a gun cleaning rod.  Cut a piece of the abrasive pad and put it through the slot in the patch holder, put the rod section in a drill-driver, and buff the ID of the parts for 20 seconds or so.  

Great advice from MickD!! Surface prep is important, particularly with minimal, split seats, with so much less contact area.

Thanks guys my other question is, is it possible to purchase the reel seat Abu uses on their Villain Rod? That's my all time favorite seat but can't find a close match.

The current Villain 2.0 looks to have a Fuji ACS reel seat. Im guessing you want the previous version. The closest you will get is likely a Pac Bay Minima split seat.

15 minutes ago, .ghoti. said:

The current Villain 2.0 looks to have a Fuji ACS reel seat. Im guessing you want the previous version. The closest you will get is likely a Pac Bay Minima split seat.

Yes the first gen was the seat I was looking for.  I assumed hat the minima was the closest but i figured it was worth asking