Most professional installers don’t work with SmartThings yet because of the reliability issues, the lack of margin, and the lack of security features. It’s still essentially a DIY product. But at least it would help to know the city where the home is located to see if there’s anyone who might be available. 
I do think most people with homes of that size go with control 4 or Crestron. What were the negatives in your Crestron experience?
I assume you’re planning to go with something else for your security system. So what specific use cases did you have in mind for SmartThings? If it’s just lighting, you might just look at a Lutron system. A professionally installed one can definitely scale to that size, devices are very well engineered and highly reliable. It’s just that they only do lighting.
As far as whether SmartThings can scale to that size, A lot depends on the actual dimensions of the house. If you want to run Z wave devices, you’re going to run into hop limits. A zwave message is limited to a maximum of four hops. In typical US residential construction, a hop for a Z wave plus device will be about 75 feet. The problem is that even if your home is a perfect cube with the hub located in the exact center, four hops with only Z wave plus devices (none of the older Z wave classic generations) would theoretically cover the size you you mention. However, if the house is spread out at all you’re going to have problems. Also, Z wave has a hard stop of 232 devices.
This is why most of the high-end systems, including control 4, use zigbee instead. Zigbee home automation allows for 15 hops in and 15 hops out at the hub and can handle thousands of devices. So even though each hop is shorter, about 40 feet, you can just get much bigger coverage with a zigbee system. In addition, control 4 has done some really cool stuff with some proprietary implementations so that they can have multiple zigbee coordinators on one install.
You can take a look at the quick browse lists in the community – created wiki, check the project report section, and look at the “whole house” list to see what some other community members have done, but I think most of the “large house” projects listed there are around 4000 ft.². I don’t remember anything in the size that you’re talking about.
http://thingsthataresmart.wiki/index.php?title=How_to_Quick_Browse_the_Community-Created_SmartApps_Forum_Section#Quick_Browse_Links_for_Project_Reports.2FQuestions
But for anyone else doing research and finding this thread in the future, 99.9% reliability is not currently a match for SmartThings.
, I will not recommend it).
I was a network engineer before I got sick and had worked with both zwave and zigbee before I ever bought a SmartThings device, so I feel pretty confident that my local network is set up well. But there’s nothing that we as individual customers can do to protect against changes to cloud operations, hub firmware, the mobile app, or stock DTHs. Those happen when they happen, and if they have glitches, we either find a workaround or wait for SmartThings to fix them.