In a lot of my work I use state space matrices for control systems design.
This is my go-to way of visualising the dimensions of matrices in a linear time invariant state space system
given by the following equations:
x = Ax+Bu
y = Cx+Du
Understanding this really helps when you're to hand-coding portions of controller-plant systems for analyses such as stability, controllability and robustness and have to do a lot of state substitutions and forget what sizes your matrices need to be to correctly multiply together!
The distribution of colour is a heat-map of sorts based on the sizes of state space systems I worked with during university projects while I was studying for qualifications in aerospace systems engineering.
As you can see, I worked with a lot of quite small SISO or few input-output systems and only a few very large MIMO systems.