I've been struggling with this for a bit now but I cannot seem to find a way to wrap either layer names or headings in a legend. I can see how to wrap descriptions, labels, and make multiline legend titles but nothing for layer names and headings.
I have a long layer name/heading as pictured below and I cannot get it to run past the width of the symbols and labels. I can think of a couple of workarounds including moving this layer to the top and putting the layer name into the Legend title but would prefer not to have this item at the top of my legend.