Changing the default font family in TinyMCE

For TinyMCE 4.6.3 this seems to be the way to go:

tinymce.init({
    setup: function (ed) {
        ed.on('init', function (e) {
            ed.execCommand("fontName", false, "Verdana");
        });
    }
});

// Init TinyMCE
$('#content').tinymce({
    setup : function(ed)
    {
        ed.on('init', function() 
        {
            this.getDoc().body.style.fontSize = '12px';
            this.getDoc().body.style.fontFamily = 'serif';
        });
    }
});

maybe too late but...

$('.tinymce').tinymce({
    setup : function(ed) {
        ed.onInit.add(function(ed) {
            ed.execCommand("fontName", false, "Arial");
            ed.execCommand("fontSize", false, "2");
        });
    }
});

EDIT

For TinyMCE 4, as @jason-tolliver and @georg states, the syntax is:

ed.on('init', function (ed) {
    ed.target.editorCommands.execCommand("fontName", false, "Arial");
});

For those who init timymce with tinymce.init({ and cannot implement Radius Kuntoro answer directly.

My init looks like

tinymce.init({
            selector: '#editor',
            menubar: false,
            plugins: ['bbcode'],
            toolbar: 'undo redo | bold italic underline',    
            setup : function(ed)
            {
                ed.on('init', function() 
                {
                    this.getDoc().body.style.fontSize = '12';
                    this.getDoc().body.style.fontFamily = 'Arial';
                });
            },
        });