Can I change the font size in the script editor?

Yes!

var _source = mywindow.findChild("_source");
var _existing = mywindow.findChild("_find");

var _layout = toolbox.widgetGetLayout(_existing);

var _fontSize = toolbox.createWidget("QSpinBox", mywindow, "_fontSize");

_fontSize.value = 10;

_layout.addWidget(_fontSize);

function sHandleFontSizeChange() {

_source.setFontPointSize(_fontSize.value);

}

 _fontSize["valueChanged(int)"].connect(sHandleFontSizeChange);