Perhaps step outside the strictly "size" box:

If confined geographically, perhaps: citywide, countywide, watershed-wide, peninsula-wide, everyone downstream of, everyone downwind of, in the X canyon, etcetera.

If defined by infrastructure failure, perhaps: system-wide, everyone dependent for [resouce] from X utility, beyond X's capability to respond, etcetera.

By comparison to specifically defined governmental unit current census known to or identified for the target audience [city, county, state, country], perhaps: San Franciso level, Sacramento County level, California level, mainland United States level, etcetera.

By comparison to specifically defined historical experience numbers, perhaps: United States polio epidemic, Spanish flu pandemic of 19??, European Black Plague of ????, etcetera.

By referencing animal population threat status terms, perhaps: at-risk, endangered, extinction-level, etcetra.

Numerically, perhaps: literal one-in-ten decimation,

Grammatically, perhaps: "extremely" large event, use of the words like "beyond, exceeding," etcetera.