Depending what exactly you need, it will be more or less difficult since this data is not exposed in a "user friendly" way (as far as I know) here is the link to the documentaiton

The SpinePlugin uses the sprite.drawDebug = true flag to show many different parts of the spine data.
In this example you can see, how this parts a divided: https://phaser.io/examples/v3/view/spine/draw-debug-bounds#

With the Buttons you can switch on and off the different parts

Depending on which group of boxes you need, you would have to access the data, similar as the plugin itself. (here is the link to the plugin source code around lines 9936 ~ 10095 ). The data should be in the property sprite.skeleton , but you would have to find the parts you really need.

Update:
if you are using physics and the physics bounding box is enough, like in this example: https://phaser.io/examples/v3/view/spine/arcade-physics-spine-body#

you could get the size of the bounding box with image.body.width and image.body.height, and you can alter it also with setSize.