PHPExcel: Setting font size?

Use setSize method instead setFontSize, it should work:

$objPHPExcel->getActiveSheet()->getStyle("F1:G1")->getFont()->setSize(16);

If you want to use the style array, then you can do something like this:

$fontStyle = [
    'font' => [
        'size' => 16
    ]
];

$workbook->getActiveSheet()
    ->getStyle("F1:G1")
    ->applyFromArray($fontStyle);