I am 6' 3" 250 lbs. Owned Atomic Big Daddy 192 (125 under foot) which worked in powder but not on the groomers getting back to the lift. My everyday board is Ski Logik Ullr's Chariot 188 (101 underfoot) which works well in not too deep powder. The line Mothership 185 (111 underfoot) is in the same class as the Ullr's as to the depth of snow it handles with me on board. Also have Blizzard Answer 185 (110) which feel small (does not float me well enough) in powder.
So this year I got the Ski Logik Depth Hoar 191 (143 underfoot) and Praxis Powder Boards 200 (138 underfoot) both of them are a great ride in powder for my size.
My point is I feel big guys need big skis! Me listening to a 150 lbs sales man tell me about his big powder days on skis smaller than my everyday board no longer holds much cred with me. I know it is tough to demo boards on a powder day but you should and try some wide ones. The depth of the snow and light/heavy conditions also have a big bearing on the experience of a particular ski. My first few days on the Big Daddy was so much better than anything I had been on before I fell in love. But after a few days of skiing them flat back to the lift got old, they had no side cut (41M).
I skied the Depth Hoars on Tuesday this week in about a foot of powder and they were great and I can carve them on groomers. The two boards listed above are certainly not everyday boards but if you want to really experience powder, it takes a specialty board (for a big man). Last season I had a bunch of great powder days on the Ullr's and the Mothership, but some days I just need more float.
Remember, big man, big ski.