Pyro instead of
eq use
== because you are editing php file not smarty file
Not tested but your code should be like this
PHP Code:
if ($product.categoryid == '1') {
$product_tabs[] = array(
'title' => 'Sizing Chart',
'tpl' => 'customer/main/sizing_chart_tab.tpl',
'anchor' => 'sizechart'
);
}