You can create a new RTE format type. Just perform Save as of "Default" RTE format and save it with some other name. For exmaple, "CustomRTEStyle". Put the styling there as you need like font size, typograhy etc.
Once you have created the format in the skin rule, Open the rule where you are using the RTE control. Go to the presentation tab of the file, and select the "CustomRTEStyle" format from the layout type option.