sounds like you need to move the seat back Aldus. I’m new to the sport, but I don’t think that there are “seat dimensions” for you – it becomes a drivers preference thing.
My understanding is that you move the seat to where it is comfortable for you, mount it, then scale it to understand the weight distribution. If moving the seat back puts too much weight in the rear then use weights in the front to regain the desired F/R L/R balance. Seems most people strive for 50/50 bias left to right, and then 52-ish/48-ish rear to front as a general rule, and then drive it for awhile and change up as more is learned.
But if you can’t fit your butt in the kart then that is the first problem to solve, tune the chassis out once you get the seat positioned and mounted to your driving preference. I’m mounting my seat today as well so this is a timely subject for me, I’m 6’3 and 235 so I’m imagining that I’ll have a big task in front of me too