The triple-slot card measures 31.3 cm in length, and is 13.8 cm tall.
31.1 cm = 12.25" long
13.8 cm = 5.43" high
The 3090 is about .5" shorter in length than my 980Ti AMP Omega, but stands about .2" taller. And she's just as wide, triple slot.
I could drop a 3090 in my CM HAF XB Evo case. I wouldn't have to worry about GPU sag. The case isn't that pretty looking, but I find it great that MB mounts parallel to the ground and my GPU sits on top; no more GPU sag for me.
Then again, the 3090 is a month's car payment, cell phone bill and mortgage.....if I didn't have any of those things I could afford one. Who am I kidding? If I didn't have those things the wife would want me to spend my money on other crap we don't need instead.
Yeah, the 3090 is a big card no doubt but it's not unprecedented. The MSI Lightning 1080Ti Z which sits in my case is also longer (12.6 inches vs 12.3 inches for the 3090) and taller (140mm/5.5 inches vs 138/5.4 inches). The Lightning Z is not as wide but though it's spec'ed as a 2-slot card, in reality it's closer to 3-slots.