I saw this K642-5605 car-
and I’m wondering, since it’s O-27 -
and its K-Line, will it be closer to Railking or MTH Premiere in actual size.
The K-Line 6xxx O-27 boxcars are the same size as postwar/MPC Lionel 6464 types, but the K-line cars have more added on details.
These are smaller than RailKing and Williams boxcars.
It seems to me that you got your answer. The boxcar you are asking about is in the K-Line 600-series, and is Railking-sized, or even a little smaller.
The 6464 boxcar, its derivatives and clones, have been made almost continuously for over 65 years now. Since most people have seen them, comparison to a 6464-type is every bit as commonplace as "is it bigger than a breadbox?" and maybe more so.
Regarding the 6" height limit, no boxcar in O is going to get anywhere near that. Boxcars just aren't that tall. A full 1:48 scale double-stack car or auto rack might get up there, but boxcars won't. If you have a 1:48 boxcar (what I believe all the MTH Premier to be), then it will, perforce, look "right" next to a 1:48 double-stack or auto rack, because both are modeled to scale. But it won't look uniform, because the real thing is not uniform.
Long story short, whatever you have that needs 6" clearance will not look right with the K-Line car for two reasons.
1. Your 6" tall cars are almost certainly 1:48 scale, but the K-Line car (being a 6464 clone) is only about 1:53.
2. Your 6" tall cars are almost certainly modeled after modern prototypes, while the K-Line car is modeled after a 40 foot boxcar, which is tiny by modern standards, and which went the way of the dodo long before modern container and auto cars showed up.