Just have a look in /usr/share/icons/Yaru/16x16/places folder in Nautilus 3.30.4 in Ubuntu 19.04, and you’d see for yourself. or 24x24, 32x32
Better use svg images, rather than png ones. They are clear and crispy. Nautilus can’t show the exact sizes as images, though. Every icon is shown as same size, but each one is of different sizes,
Real size
(Actually, this post should go there. )