Bus Connector size? (not 2.54)
What is the actual size of the bus header as it is too small for my Dupont 2.54mm pins?
I gather you are talking about the internal MBUS GPIO pins - that connect the modules together.
The pin pitch - distance between the pins - is 2.54 but the pins are height reduced compared to those found on a standard GPIO header - too SHORT for Dupont connectors to attach.
The MBUS pins are intentionally height reduced in order to minimise the overall height of the M5Stack.
It is possible to source a third party full height 30 pin GPIO header if needed, or you can make one using a GPIO for Arduino or Raspi. You could get a PCB made to go with it, based on the files available on github, see here.