this feature has been removed in Royal TSX 4.0 and we have no plans at the moment to bring it back.
Since connections automatically adjust their content to fit the available space by default, we feel like this option caused more issues than it did any good. Also, with the new split tab mode in Royal TSX V4 and feature like being able to automatically hide the navigation panel, toolbar, etc. it became very hard and error prone to further support this feature.
Seems that the Royal TSX Preferences > User Interface > Automatically Resize Windows to Session Size option is missing in 4.x. Could it be somewhere else now?