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%