From the Session Management screen where users go to request and connect to their sessions. Next to the blue "Connect" button, there is a drop down menu which lists the available screen resolutions that you can choose from.
The resolutions available from the PSM applet are:
2560 x 1440
1920 x 1080
1600 x 900
1366 x 768
2.5.919 and above adds the "Fullscreen" option. It is possible to toggle between Fullscreen / Windowed option via the Controls Menu | Session Controls. Once a user with a particular resolution has connected to a Fullscreen PSM session, that resolution will be added to the list.
Enhancement request number BFER 8882 (Ability to configure a global default resolution other than 1024x768 / Fullscreen) has been submitted to Development for consideration in a future version