There is one thing that I do remember. In one of the essentials adventures there is a rule that small characters can squeeze through an arrow slit.

There is one thing that I do remember. In one of the essentials adventures, "Reavers of Harkenwold", there is a rule that small characters can squeeze through an arrow slit.

Embrasures: The area's "windows" are arrow slits, only about 6 inches wide. Creatures adjacent to an embrasure have superior cover against attacks through the embrasure. A Small character can squeeze through an embrasure with a DC 21 Acrobatics check.

So you could argue either way with being able to squeeze through arrow slits, because an Embrasure is a kind of arrow slit.