To get 2 lines put a '\n' where you want the line break: "Q1 BTI vs, POR\n(with open po's from 2015)"
To change the font size of the second line, I use the render event to get the text element of the second line and manipulate it. Set the x coordinate to the center of the chart, change the text-anchor to middle and then get the second element and change its font size:
$("#chart").kendoChart({
title: {
text: "Q1 BTI vs, POR\n(with open po's from 2015)",