Why is an integer variable in PHP interpreted as a string literal when passed from Drupal to JavaScript via Drupal.settings?
If you do

{
  ..
  height: settings.designer.jquery_modal_height,
  ..
}


You are doing it right, else you should do

..
var height = settings.designer.jquery_modal_height;
..


Most likely it's not the variable that's the problem but some other part of your code.