How do you figure out what size reel seat to purchase for a specific blank without having the blank in hand.  For example a SCIII 7'MM with an ECS seat

Call whoever you are buying it from, tell them where you want it mounted, and they will hook you up.  

Hint: Have them throw in the next largest and smallest size.

Anthony

just tell who ever your buying it from how long of a handle you want, and you want the seat fitted there.

I always tell them I want the next size smaller, that way I can ream it to fit.

The best way is to measure the diameter of the blank where you want the seat to fit and buy accordingly.  You also can usually estimate the blank diameter by using the butt diameter and "guessing" the reduction in diameter with the taper-it will likely be very close to an available reel seat inner diameter.  I agree that it it better to be a little tight and ream rather than too loose and shim.