Is there a minimum height for a small sub-panel?
There's no minimum height, but it can't be higher than 6'7&quot; at the highest point of the &quot;operating handle.&quot;
The code specifically states:

240.24(A) Accessibility. Circuit breakers and switches containing fuses shall be readily accessible and installed so that the center of
the grip of the operating handle of the switch or circuit breaker,
when in its highest position, is not more than 2.0 m (6 ft 7 in.)
above the floor or working platform...

You also need adequate working area in front of and to the sides of the panel and be able to open the panel door to at least 90º.