I recommend using the latest available version of the Telerik UI for ASP.NET AJAX to resolve the issues.
The RadEditor control is greatly improved about its rendering and know provides more consistent behavior across browsers. These improvements are quite complex and I cannot provide a simple workaround to resolve the layout issues.
About resizing the control according to the browser, I can also suggest using a later version of the controls to improve using of editor dimensions with percent values. Also I can suggest following this help article
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