There is two places in mirc.ini where you might find the width of the nicklist. Below wil grab the non-default if it exists, or the default if it does not.
//echo -a Nicklist width: $gettok($gettok($readini($mircini,nicklist,#),2,44) $gettok($readini($mircini,options,n4),29,44),1,32)
You can use it in a timer to echo the size, but only when you drop (release the mouse button) it. Try it and you'll see what I mean.