module change values

Good afternoon to everybody.

i'm trying to find out how to solve a problem

i've a form with several dropdownlist for clothes,and the point is :

if in the first dropdown list  (gender) you choose  if you choose female,the second one (size) should have XS,S,L. on the contrary if you choose male,the second one (size) should have these values R,L,XL,XXL

and a third one (skirt size) should be not visibile.

i know (thanks to this forum) how to make invisible a module,but how i can change its values ???

thanks to all.

fabio