Here is the solution for this problem:
$html2pdf = new HTML2PDF('P', array($width_in_mm,$height_in_mm), 'en', true, 'UTF-8', array(0, 0, 0, 0));
Width and Height should be in MM. If your using inches convert it to MM.
Formula:
$width_in_mm = $width_in_inches * 25.4;
$height_in_mm = $height_in_inches * 25.4;
Don't round it off. Used the exact conversion even if it has a decimal point.
Hope this answer will solve your problem.