In the properties of the sprite, there is the Scale property, when it is in (1,1) it means that it maintains the original scale of the sprite texture. (0.5, 0.5) would reduce the size of the sprite by half. If: scale (1,1) -> 32x32px, then: scale (0.5,0.5) -> 16x16px.
If your question is about modifying the original texture size, as far as I know, it has to be in an external image editor.