This sentence "Cómo será de feo, que es más feo que lo de las mujeres" is from the novel "El amor en los tiempos del cólera" by Gabriel GarcíA Márquez. I understand that it means "How ugly it is, even uglier than a woman's thing". My question is on the grammar.
"De" as a preposition should be followed by a noun or something like a noun, for example, "Soy de Barcelona", so I do not understand the "de feo" part. Also the sentence is, when translated to English, "How it will be of ugly", which does not make sense.
If you need context, you can find it here: https://books.google.com/books?id=nui9DwAAQBAJ&pg=PA188&lpg=PA188#v=onepage&q&f=false