TCPDF Custom page size

No editing of the class is require... tcpdf doesn't accept a width/length parameter, it just accepts two lengths and determines which is which using the layout (either Portrait or Landscape)

$pageLayout = array($width, $height); //  or array($height, $width) 
$pdf = new TCPDF('p', 'pt', $pageLayout, true, 'UTF-8', false);

Go to /config/tcpdf_config.php and around line 117, modify the line:

define ('PDF_PAGE_FORMAT', 'A4');

by

define ('PDF_PAGE_FORMAT', 'LETTER');

It is important to put "LETTER" in uppercase, you can see all possible values in this file: tcpdf/include/tcpdf_static.php.