In misc.survivalism Daniel Prince wrote:

How small of an opening can they get through, 0.5 inches, 0.25
inches, 0.125 inches?

FWIW, I was once told that a hole the diameter of a pencil is all they
need.

--
The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts.
-- Bertrand Russel