The word world-spanning is formed by analogy so that it, and its analogues, are not always going to be found in a dictionary. The empire is one that spans the world.
Such compounds can be pretty much coined as needed, though some are "ready made" collocations.
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tree-hugging commies
earth-shattering news
mind-numbing repetition
tongue-twisting syllables
fire-extinguishing foam
itch-soothing ointment
frequency-modulating device
hair-raising adventure
back-breaking labor
spine-tingling suspense
news-dominating soundbite
mind-altering substance
thirst-quenching beverage