in my case i needed to manually adapt the height of an outer container that contained a background NinePatchRect, so with just waiting for the _process I could catch the size change correctly, but then I had to change the size of the NinePatch, which took another drawcycle to be done and would not be shown immediately.
What DOES work actually right away is getting the number of lines after setting a new text for your label!
So in my specific case (ONLY vertical growth, now width change) I could just use getlineheight() and multiply it by label.getlinecount() RIGHT AFTER I set the text with label.text
p.s.: actually, in my case the getlineheight() did not give me the correct value, it reported the line to be 3 pixels lower than it actualy was, not sure if that has to do with some setting i used for my label, so i ended up using fixed line_height values in my script and adding a padding for good measure.