all over; throughout; everywhere; from head to toe; from start to end
Emphasizes covering every part of a place, object, or range; often used with verbs like 探す, 調べる, 知っている.
I searched every inch of the room, but couldn't find the key.
彼はこの町のことをどこからどこまで知っている。
He knows this town inside out.
A set phrase composed of どこ (where) + から (from) + どこ (where) + まで (to), literally 'from where to where', used idiomatically to mean 'every part' or 'all over'.