Australian slang used to exaggerate size and imply a negative conotation. Usually considered a derogatory term and is most often used in relation to someone who is morbidly obese.
To be grammatically correct the term may either be:
placed immediately before a size related adjective e.g. That port-a-loo is vellis big; or
used as an adjective e.g. He's so fat, he's vellis.
Terms is possibly derived from large beast described in ancient Greek mythology.
That guy is so vellis fat he's not going to fit through the door.
His Subway diet is two feet of meatball sub and he wonders why he's vellis massive.