Please use the 'onExpand' event of layout. It is triggered after expanding a region panel.
$('#layout').layout({
onExpand: function(region){
var cp = $(this).layout('panel','center');
var width = cp.panel('panel').outerWidth();
console.log(width)
},
onCollapse: function(region){
var cp = $(this).layout('panel','center');
var width = cp.panel('panel').outerWidth();
console.log(width)
}
})