丈 is an old unit of about 3.3 metres, so
万丈 'ten thousand zhang' is a poetic way of saying immeasurably high, deep or long. Before a noun it gives height or depth, as in
万丈深渊 'a bottomless abyss'; after a noun, boundless light or feeling, as in
光芒万丈 'shining far and wide'. The proverb
万丈高楼平地起 'the tallest tower rises from level ground' says that every great thing starts from the bottom.
