Diameter of an object = the maximum distance between two points in the object
It is the length of the major axis.
The measurement isn't stable under noise to be very useful for anything other than very nice objects. For more on the issue, see Robustness of geometry.
This measurement isn't implemented in Pixcavator. Thickness and length is a good substitute.
To experiment with the concepts, download the free Pixcavator Student Edition.