While minimalism is great, there is a point of diminishing returns. If you set your icons to 16x16 pixels (the absolute minimum on most OSes):
Mac users have fewer granular options, but the results are clean. reduce icon size on desktop
Click an empty spot on the desktop, then hold the key and scroll your mouse wheel down to shrink icons to any custom size. Keyboard Shortcuts: While minimalism is great, there is a point
If the smallest standard setting is still too big, you must change your Screen Resolution scaling: While minimalism is great