Maybe you could do it on terrestrial vs arboreal such as a panther or veiled is aboreal and leaf chameleons are terrestrial you could also mention that terrestrial species are generally smaller then aboreal species hope this helped!
its not completly the hight thats the problem its the fact that they dont want me spending tons of money on a 'Lizard' when really I am very passionate about them and love them and wish my parents were more supportive