Working from memory, the PI box is about 18d x 20w x 8-12h. You can load 19" stuff N/S if you rest it on the firebrick but you may have dead space underneath. The thickest piece you can stick in to fall into the bottom of the firebox is 8", but the total height of the firebox is maybe 12", increasing to maybe 16" in front of the cat. (I don't use that front-of-cat area anyway, because of flame impingement and because wood might fall onto the door glass.). You normally don't have the full height to work with unless you are a diligent ash shoveller.
These numbers may all be off by a little because I didn't measure them. (Fire is hot and all that.)
The Siccoro insert is supposed to be ~0.5 cf smaller than the princess, but I've never seen it. If it's only 0.5 cf smaller and flush, the engineers must have got their shoehorns out for the occasion.