i've found a work around for this situation
it seems that height is directly proportional to line spacing so you just have to apply the same factor to image height
for example :
$spacing = 0.7;
$params = array("linespacing" => $spacing);
$box = imageftbbox ($size, 0, $font, $text, $params);
$tw=$box[4]-$box[0]; //image width
$th=($box[1]-$box[5])*$spacing; //image height