Increased projectile velocity when holding button down 2D

I’m new to Unity and trying to make a Worms type game in 2 or 2.5d. So far, I have my script for shooting working, but it only shoots at the set velocity. I need to figure out how to increase the power based on how long the shoot button is held down.

Any help would be greatly appreciated. Thanks!

My current shooting code:

var Shell:Rigidbody;
var mag:float = 10;

function Update () 
		var newShell:Rigidbody;
		newShell = Instantiate(Shell,transform.position,Quaternion.identity);
		newShell.velocity = transform.TransformDirection(mag,0,0);

Small example:

var force : float;
var increase : float = 1;

if (input.GetMouseButton (0)) {
    force += increase*Time.deltaTime;

if (input.GetMouseButtonUp (0)) {
    //your shoot function
    force = 0; //reset the force