I have two 6x8 platforms in the gym. They are plenty big for everything we do.
One of my plans this Spring is to build a garage gym. The back wall of my garage has a heating unit near the ceiling which would interfere with a rack, so I'd need to mount the rack on the end of the platform away from the wall. The garage also needs to be used to store a car, at least part of the year (winters in PA). An eight foot deep platform wouldn't leave room for the car (or if I moved the rack back from the end, the car would be on the platform). I can manage a six-foot deep platform though (still eight feet wide), is that deep enough? I'm mostly concerned about the bench-press, the bench is four feet long, if I mount the rack maybe six inches from the end of the platform (again, the end away from the back wall) I should be able to get the bench in far enough to get under the bar and still have room for my feet between the bench and the wall, shouldn't I? The alternative would be a two-piece platform where I'd remove one piece when I need to park a car there. I'd deadlift inside the rack.
I'll probably stick with a 24" deep rack (I've been looking hard at the Titan X2), and probably a plate tree on each side of the rack.
I have two 6x8 platforms in the gym. They are plenty big for everything we do.
Position the rack so the car straddles the part that sticks out for your bench