How do I know if my range hood is actually powerful enough for this 66-inch setup?

Marble is notoriously bad at handling thermal shock, so your concern about the backsplash is valid. When you have a 66-inch range, you're likely putting out anywhere from 90,000 to over 120,000 BTUs if you have multiple burners and the griddle going. If that marble gets too hot too fast, it can acutally crack or discolor over time. You need to check the distance between the cooking surface and the bottom of the hood insert. For a pro-style range of that size, you realy shold have at least 30 to 36 inches of clearance, and the hood itself should ideally be 6 inches wider than the range to create a proper capture area. To test the airflow without professional tools, try the smoke pencil test or even just a stick of incense. Light it and move it around the edges of the range while the fan is on high. If the smoke is being pulled toward the center and up into the baffles, the CFM might be okay, but if it's drifting out into the room, your capture area is too small or the blower is underpowered. Most people install a 1200 CFM blower for these big units, but if the ductwork has too many 90-degree bends or is too narrow, you'll lose half that power before it even reaches the roof. And honestly, Viking and other high-end ranges are basically industrial heaters. You're going to feel heat no matter what. The sauna feeling ussually means you don't have enugh **makeup air**. If your hood is sucking out 1200 cubic feet of air per minute but your house is sealed tight, it creates a vacuum and the fan can't actually move the air. Open a window in the next room while you're cooking; if the heat suddenly clears out and the fan sound changes, you definitely need a dedicated makeup air system installed. Don't freak out yet, but do pull the filters and check the model number on the blower motor. If they put a 600 CFM unit in there just to save on ducting costs, you'll never be able to use more than two burners at once without roasting your kitchen. You spent too much on the stone to let a cheap ventilation setup ruin it.

I ran into this exact thing with my 48-inch setup. The builder tried to get away with using 6-inch flex ducting, and the heat was just bouncing off the wall. I ended up ripping out the soffit and swapping it for a 10-inch rigid pipe. It made a massive difference in how much heat actually got sucked out vs just swirling around the marble. If you’ve got a 66-inch range, anything less than a 10 or 12-inch duct is going to choke that motor. Check the pipe size before they finish the drywall; it's way easier to fix now than later.

Check your hood depth immediatly. alot of these custom stone surrounds are built way too shallow for a pro-range footprint. If that marble isn't at least 27 inches deep, your front burners are basicly bypassing the capture area entirely. I’ve seen contractors slap a Best by Broan PIK series insert into a beautiful 66-inch box but mount it too far back. Also, take a look at the filters. If they're mesh instead of heavy stainless baffles, they’ll choke out a 1200 CFM blower in minutes once the grease hits. If that insert isn't sealed perfectly to the transition, you're just baking the inside of your stone surround.