What you need to do is set the Height
property by modifying the respective ConfigFile.xml or ListConfigFile.xml (forms - lists, wiki, blog ...), which are located in Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\188.8.131.52__1f131a624888eeed\Resources folder.
For example add the following code.
I hope this helps.
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