Changing the width of a stack

[Peter Haworth]

I have a stack with three controls on it, one at the left edge of the
stack, one centered horizontally, and one at the right edge of the stack.

I want to change the dimensions of the stack so that it's left edge is at
the left of the centered control and its right edge is at the right of the
centered control. And also change its dimensions back to the original
settings when necessary.

As I have a graphically challenged brain, I have been unable to figure out
how to do this.

References to the left of a  stack are relative to the edge of the screen
so they move the position of the stack on the screen and don't change its
width.

I'm probably missing something obvious and hoping that someone will point
it out to me.

Pete