There was a post on Epic (and copied here) at one time by PhysicsMan about the different surface area needed by lighter skiers versus a 200 pound guy.? My aged memory is that 70mm for 120 pounds is pretty equivalent to 100mm for 200 pounder.? Of course, there is a lot more that goes into the real ski feel, but I would imagine that mid-70's to low 80's would be an excellent soft snow ski for Terryl.
Lynn is slightly shorter than Terryl and weights less than 120 and skis the Head Every Thangs in a 156 (72mm waist) and has skied up to about 10" of powder in those, but they are really her EveryThing (name pun intended) ski and not a West coast only ski.? She also demoed the Fischer Vision Vapor (76mm waist) and really liked that but was concerned that 76 mm was a bit wide to bring back to the east.? Ultimately liked the Every Thangs more (and hasn't second guessed that decision a bit).? If she were to buy a soft snowski at this point in addition to the Every Thangs, I think she would be looking at 78-84mm waists.
I would assume that 165-172 would be good lengths for Terryl's size.? What length is she skiing the Speeds at?
However, I see little reason why you would pay much attention to east coast versatility in the Mythic Rider, just so you can use it on your local hill.?

? You've got an excellent ski for that; focus on picking the ski for the task at hand.? You don't want/need a quiver of "do it everywhere" skis.? ?

? FWIW, Greg is very much your size and in a ski where float and surface area is important I would give his recommendation a lot of consideration.? It's soft enough that you will still be able to turn it, and the extra few centimeters are probably worthwhile. (and, as you know, Dawgcatching is a good knowledgeable guy as well)
PS. It's wonderful that you 'hit' a snow dump week; that doesn't happen to me very often, certainly less than one trip in four.