Regarding fit - I have a 2nd-gen Kingdom 8. I've fit my family of 5 + family dog + gear in there with a combo of a cot and air mattresses. I've fit 3 grown men with their own cots + gear. I've fit 2 men with cots + gear & a chair each in there. I'm 6ft2 and I still have an inch of headroom to spare standing up.
Regarding wind stability - I've never had a problem even with wind. However you MUST think about which way the wind direction is coming from when setting it up. Like boating, you want to point the bow of the boat into the waves, and same with the Kingdom tents & wind. You definitely do NOT want to have wind blow broadside into the tent! Also, ALWAYS use all your extra guy-out points in windy conditions, no matter what direction it comes from. You can't ignore that without risking serious damage to the aluminum poles.
Regarding waterproofness in rain - Again I've never had a problem. Bone-dry every time, even in snow. No drips at all, unlike what I've heard about the 3rd-gen Kingdom tents. I also use the optional footprint, which aids floor protection in durability and keeping moisture out. Another good tip - After you setup with the rain fly, get some "camp-dry" waterproofer sealant and spray the fly with a good coating & let dry. I usually do that once a year.
I love this tent! Well worth the investment.