height of editordocument

Try:

function FCKeditor_OnComplete( editorInstance )
{
alert(computedStyle(document.getElementById('FCKeditor1' + '___Frame'), 'height')) ;
}

// get computed style property of an element
// cssProperty is the javascript property notation (e.g. "backgroundColor") and mozillaEquivalentCSS is the CSS notation (e.g. "background-color")
function computedStyle(htmlElement, cssProperty, mozillaEquivalentCSS)
{
if (arguments.length == 2) {
mozillaEquivalentCSS = cssProperty;
}
if (htmlElement.currentStyle) {
return htmlElement.currentStyle[cssProperty];
}
if (typeof(document.defaultView) == 'undefined') {
return undefined;
}
if (document.defaultView === null) {
return undefined;
}
var style = document.defaultView.getComputedStyle(htmlElement, null);
if (typeof(style) == "undefined" || style === null) {
return undefined;
}
return style.getPropertyValue(mozillaEquivalentCSS);
}

And maybe see the FitWindow plugin. This is an example on setting the editor its height.
Like document.getElementById('FCKeditor1' + '___Frame').style.height = "999px";