Basic example

jsPanel.create({
    content:     '<textarea></textarea>',
    contentSize: '750 400',
    headerTitle: 'jsPanel with TinyMCE',
    theme:       'primary',
    position:    'center-top 0 70',
    callback: function (panel) {
        tinymce.init({
            selector:'.jsPanel-content textarea',
            height: 288 // contentSize.height - 112
        });
    },
    resizeit: {
        resize: function (panel) {
            resizeEditor(panel);
        }
    },
    onmaximized: function (panel) {
        resizeEditor(panel);
    },
    onnormalized: function (panel) {
        resizeEditor(panel);
    }
});

// function to resize the editor area whenever panel is resized using mouse or maximize/normalize methods
function resizeEditor(panel) {
    var h = parseInt(panel.content.style.height) - 112;
    $('.mce-edit-area', panel.content).css('height', h);
}