Untitled

want to decrease the size of the div which is added in dialog
$(function() {
        $( "#maincontainer" ).dialog({
            autoOpen: false,
            width:50%,
            height:400,
            autoClose: false,
            show: "fold",
            hide: "core"
        });

        $( "#previewBtn").click(function() {
            $( "#maincontainer" ).dialog( "open" );
            return false;
        });
});