Dbu vs. Pixels
The dbu option makes mIRC use dialog base units when creating the dialog, this ensures that the dialog will look the same for all users under any size display, etc. Pixels is the default however to maintain compatibility with previous scripts.
The identifiers $dbuw and $dbuh return the dbu per pixel width and height values for the current display. This may or may not be an integer value.
dialog -s $dname $dialog($dname).x $dialog($dname).y $calc($width(%text,%font,%fontsize) * $dbuw) $dialog($dname).h