GetSize

<div id="fileManager"></div>
<script>
    var baseUrl = "https://demos.telerik.com/service/v2/core/filemanager/";

    $("#fileManager").kendoFileManager({
        width: 800,
        height: 600,
        dataSource: {
            transport: {
                read: {
                    method: "POST",
                    url: baseUrl + "Read"
                }
            }
        }
    });

    var fileManager = $("#fileManager").data("kendoFileManager");
    var size = fileManager.getSize();

/* The result can be observed in the DevTools(F12) console of the browser. */
    console.log("Width: " + size.width + ", Height: " + size.height);
</script>