Massive in extent, it covers large parts of the planet, and in duration, could last for weeks or months.
The threat to Opportunity isn't that the storm could rip it apart. It's that even a low-pressure wind can lift a lot of fine dust which blocks out the sun and leaves the rover without power from its solar panel. If it lasts too long the rover could literally freeze to death.
NASA has an image from another major storm in 2007 [1], it blocked 99% of the sunlight. This one is worse.[2]