To define "look" you must say "look how", i.e. with what sense are you looking.

For atoms you can use gravity to look at them, or the electromagnetic force, or the strong force. And the atoms will look different each way.

I suppose you could define look as "how strongly will this test particle interact with the atom at this distance, using this force". But notice "how strongly" - there is no fixed boundary, the interaction just gets weaker (or less likely) as things get farther.