Is there some way to decrease folders/icons size in Nautilus 3.18?

This setting is now a little hard to find in the GUI since it is not explicitly labeled.

Click on the icon view button to bring up settings dialog; then move the slider to the left to change the icon size to small.

You can find the same setting in dconf-editor through:

org >> gnome >> >> nautilus >> icon-view >> default-zoom-level: [small, standard, large]

There is also a thumbnail-size setting that is supposed to work along with the small setting; but nautilus does not seem to follow this setting and it did not have an effect. You may want to file a bug report.

Use:-
Ctrl+ To increase the font size
Ctrl- To decrease the font size

(Press and hold the Ctrl key, then lightly tap the + key, then release both)