Dwarf refers to a person with one of several varieties of a specific genetic condition called dwarfism. A dwarf has disproportion of body parts. This is generally caused by a genetic or nutritional disability. Any adult human below the height of 4'10 (147 cm) is considered a dwarf.
One Piece has a great story and does not have a lot of filler episodes.
One piece filler episodes are generally good. I once thought the G-8 filler arc was canon because it blended in with the main story so well. One of the reasons the filler episodes of one piece are generally good is because the producers first ask the mangaka(Eiichiro Oda) for permission.