Add a scaled field to your Game class.
IsTouched(), and toggle
_scaled if it returns
_scaled = !_scaled;
Draw, use the overload of
SpriteBatch.Draw that takes a scale parameter. If
false, use a scale of 1.0f, otherwise use a scale of 2.0f or whatever scale you want. Use the origin parameter to set the origin to the centre of the sprite (half sprite width and height) so it scales equally left/right and up/down. If the origin is set to the default top/left, the sprite will only scale to the right and down.