They look a little bit like the kimono dragon but with spiny things around the head the one Ive seen he's about 18" long maybe 20" with a head width about 4".
There are some that can stretch as much as 4 feet long but are considered an endangered species so you might not be able to get one that long. There are some more common ones that are 2 feet long.
Pogona vitticeps is the largest and most commonly kept species of Bearded Dragon. There is a morph called the "German Giant" that is generally quite a bit larger than your standard dragon. Keep in mind females are usually bulkier and heavier than their male counterparts.