5 ALONG.

4 ACROSS." It was the measurement most likely of something, and he was afraid of forgetting it; therefore it was something important.

Query--something about himself?
Say "5" (inches) "along"-- he doesn't wear a wig.

Say "5" (feet) "along"-- it can't be coat, waistcoat, trousers, or underclothing.
Say "5" (yards) "along"-- it can't be anything about himself, unless he wears round his body the rope that he's sure to be hanged with one of these days.

Then it is _not_ something about himself.