The width and height values are hardcoded in the Flash dialog and you can remove them by registering the external dialog files of RadEditor and replacing the following lines
this._newImageWidth.value = "150";
this._newImageHeight.value = "150";
this._newImageWidth.value = "";
this._newImageHeight.value = "";
I examined the code of the FlashManager.ascx file and noticed that it does not contain any code, which obtains the dimensions of the selected flash object in the content area and updates the Width and Height textboxes.
If you would like I can log this as a future request and if possible we will implement it for a future version of the control.
All the best,
the Telerik team
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