Anyone have tips on selecting cummerbund size from ferro concepts. Looking at getting the 3AC cummerbunds and 3AC Cobra Buckle Kit. Not sure if I am a medium or large. Feel like I would be a medium.

Then you probably are a medium

Most plate carrier companies suggest starting with the smaller cummerbund.
Edited for speeling.

I like a tight fit, especially with a structured cummerbund. I have the medium I use for three seasons and the large I use if I need to put on my bulky winter layers.  

I have actually moved away though from the Ferro Cummerbunds and now run an AXL Equinox Cummerbund with a VXV grid adapter and AXL tubes adapters.  AXL is soon coming out  though with their own new grid adapter for the Equinox Cummerbund (might be out already).  I would highly recommend the Equinox over the AXL for the FCPC V5.