“Sheol has enlarged its appetite.” This is the figure of speech personification. Sheol is pictured as a person who cannot get enough to eat, and many people are dying and thus becoming “food for Sheol.”
“beyond measure.” This continues the figure of speech personification. So many people are dying that Sheol has opened her mouth wider than can be measured so that she can eat all the dead people.
“dignitaries.” The Hebrew is “splendor,” but in this context it refers to the dignitaries, nobles, or leaders in Jerusalem.