Best way i know to modify size of the sprite is to change the scale of the actor with the spriteRenderer (considering that it would also scale its children). So the way to go would be:
X.setScale(1, 1, 1);
Where the scale will be in pixel units, if you have setting 64 pixel units and want the sprite to look like if it where 32 pixel unit then the scale might be 0.5. (dont take my word here, its what i imagine happen)
The other way is changing the pixel unit property on the sprite editor, that way the sprite will always render at the equivalent size.