Possums can jump but they almost never do.They usually hiss and bite or play dead. They are Great at climbing. They have long skinny tails a bit thicker at the base. Possums are as big as a cat. I would guess it was a big rat you saw. Rats are faster and can appear and move very fast. If you mean huge as meaning long… rats have proportionately longer tails than opossums. An adult rat is bit less than half the size of an adult opossum.