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 () 
 {
     if(Input.GetButtonDown("Fire1"))
     {
         var newShell:Rigidbody;
         newShell = Instantiate(Shell,transform.position,Quaternion.identity);
         newShell.velocity = transform.TransformDirection(mag,0,0);
         newShell.transform.Rotate(0,90,0);
     }
 }