If you are editing the most usual content element, that is the bodytext column of tt_content table, try to add the following line to your Page TSConfig:
TCEFORM.tt_content.bodytext.RTEfullScreenWidth= 100%
Note that this setting is now included in the default configuration of the extension.
If editing some other column, use the same model:
TCEFORM.my_table_name.my_column_name.RTEfullScreenWidth= 100%