As to holding that setting, I just drag the vertical scroll bar that's between the nicklist and the channel to the postion that I want and it stays there, even if I close the channel and then re-open it.
But how do you make ALL channels have a set value by default? Dragging that vertical scrollbar seems to only change the value for that channel and not all channels. I believe it puts the value in mirc.ini under [nicklist], like so: "#channel=right,176"
What I want to know is how to be able to alter the width of all channels simultaneously, with whatever value I choose?