One into sin with demon changing size (9)
I believe the answer is:
dimension
'size' is the definition.
(I know that dimension is a type of size)
'one into sin with demon changing' is the wordplay.
'one' becomes 'I' (Roman numeral).
'into' means one lot of letters goes inside another.
'with' is a charade indicator (letters next to each other).
'changing' indicates anagramming the letters.
'sin' put after 'demon' is 'demonsin'.
'demonsin' with letters rearranged gives 'dimenson'.
'i' put into 'dimenson' is 'DIMENSION'.
(Other definitions for dimension that I've seen before include "Proportion" , "Length or breadth, say" , "measure" , "size" .)