You say you want it to grow and shrink but you don't specify when\how it grows and when\how it shrinks. If you want to grow on Fire2 and shrink on Fire1 then you'll need code like
var f1 = Input.GetAxis("Fire1");
var f2 = Input.GetAxis("Fire2");
if (f1 > 0)
{
transform.localScale += new Vector3(0f, 0f, -0.01f * f1);
}
else if (f2 > 0)
{
transform.localScale += new Vector3(0f, 0f, 0.01f * f2);
}
As well as having different conditions for growing and shrinking I've multiplied the axis value by 0.01 as the fire buttons return 0 or 1 and growing by a factor of 1 is quite extreme. Technically you could just hard-code the value as multiplying by 1 does nothing, but I'm assuming controls with analogue fire buttons like the xbox and ps4 might return a value between 0 and 1 depending on how far it is depressed.
If you want the sprite to "bounce", ie to grow to a certain size then shrink again and so on you'll need something like this;
private float scaleChange = 0.01f;
void Update()
{
var f2 = Input.GetAxis("Fire2");
if (f2 > 0)
{
transform.localScale += new Vector3(0f, 0f, scaleChange * f2);
if (transform.localScale.z >= 1.5 || transform.localScale.z <= 0.5)
{
scaleChange = -1 * scaleChange;
}
}
}