1. "Leavemessage window" has the same size as the normal chat window. Its size is defined in the button code.
see libs/operator.php, function generate_button(): .... menubar=0,width=640,height=480,resizable=1.....
2. "Mail window" has theme specific size.
see styles/<theme>/templates/chat.tpl, search for "ForwardMail"
PS: I have in my plan for the upcoming release to move all these settings into theme configuration file.