About 20.000 TEU is the current limit with "standard" LxB dimensions of 400x60 m, and some creative container stowing.
In 2008, Korean STX had developed projects for 22.000 TEU ships, having length of 460 m, but keeping the 60 m / 23 rows limit due to crane outreach limitations.
Having such a length (15% increase in length over only 10% increase in capacity over 19/20.000 TEU vessels) suggested that back in 2008 STX went for smaller block coefficient / more streamlined and less boxy hull, designed for speed. Ah, those times when people thought that double digit growth can be obtained year after year indefinitely...
(Why do you think MAN B&W in those days offered main engine designs of ... wait for it ... 18K98ME-C, and 14K1080ME-C...)