A 1/4" (6mm) is too small for a mainsheet, and you'll regret having it that small in heavy winds (or your crew will!). 5/16" is the size I'd go (8mm), and use a line like FSE Robline's "dinghy control" line, which is taperable and has a very nice coated dyneema core. The line is available through Murrays. Look up their website, call, and ask to talk to Steve Murray. He'll set you up with the line.
Another option is to splice 3/16" (5mm) spectra into a line like 5/16" (8mm) Maffioli or Robline Racing sheet. I am a bigger fan of the racing sheet as it tends to hold its shape much better and last longer.
Never ever ever cleat on the small 12 strand stuff. It will slip out when you least expect it!
*and don't get McLube on your cam cleat either*
If anyone wants to do any of this rigging, I can build anything you'd like and ship it to you.
ddbrown (at) ncsu (dot) edu