The WKA established measurements for what we call "flat" karts today, decades ago. People have pretty much stuck with them since, although other regulating bodies can (and sometimes do) make their own.
451.5 OVERALL MAXIMUM WIDTH: 50.0” for all classes.
451.6 OVERALL MAXIMUM LENGTH: 74.0”.
451.7 HEIGHT: Maximum of 26.0”.
Those "451.x" numbers are rule numbers, in the WKA Technical Manual.
I haven't found a specified measurement for king pin to king pin - that could be a parameter that chassis designers are allowed to play with.