You can use a light microscope to visualize things down to about 500 nm, somewhat dependent on shape (you can see down to about 100 nm, but really only that something is there - whereas nanocubes with 200nm side length had some definition when I tried it).

Which is pretty shocking when you realize a nanometer will hold about 10 hydrogen atoms (give or take the fact that the electron cloud repulsion isn't well defined), meaning you're in fact only talking about 5000 hydrogen atoms at the same packing for that 500 nm linear distance.

Which puts the idea of "Plenty of space at the bottom" by Feynmann in an odd contrast because it's like...well, that doesn't seem so far away when my basic eyeball and some optics can operate plausibly see down to that scale directly - photons are interacting with things and passing straight to it.