Mass Manipulation (Also called Size Manipulation) is the ability to control the size of the user's body, making themselves incredibly tall or very small. This may be done through a variety of means, the most obvious being cellular manipulation, through which cells are rapidly cloned, increasing mass, or rapidly deteriorated, decreasing mass. Another means might be that the cells themselves actually grow, or shrink, according to adrenaline.
Obviously, this ability may also result in a increase in strength and/or durability, as someone the size of the Statue Of Liberty likely will not be felled by a single round from a 9mm. Someone the size of the Statue Of Liberty would also be able to literally crush things (and people) of a normal size. As one may guess, increasing mass becomes heavier. As one may guess, decreasing mass means that there is less mass to be seen, and someone the size of an ant would be difficult to spot.
A more advanced user of this power may also be able to change the sizes of other objects other than themselves. Perhaps they literally adjust the molecular state of their companion, which would allow room for fatal error. Also, the process may be slower, and even painful, if attempted by one not fully in control of their power.