how to display overlay width and height
You should set your path in admin/config/system/overlay_paths_ui whit pattern like this node/add|600px. (I test it by myself, it effect after I clear cache in admin/config/development/performance  ).
This module cant help you to set a height for overlay.(you can patch this module to provide your require   ).
Final and good solution for do your job is use css, set css in your css file

#overlay {
height :xxx px; !important
width :xxx px; !important
}