Currently it is possible to resize windows and select resolutions on the clients when using mobile and HTML5 client. But users of other clients are locked into the resolution the admin sets, or the "Use available space". It is up to the admin to define it. In my case, I have to setup two different published desktops for the same server for "Full Screen" and "Use available space". Once the user opens the "Full Screen" it cant make it "Use availabe space" or vice versa, without manually disconnecting and relaunch it from the client with the different setting. Other competing products allow users to leave full screen and resize the desktop while on session. I understand this may be a limitation of RDP (but am under the impression RDP 8.1 supports dynamic resize) but you could easilly implement this the same way you do with mobile clients, by doing a reconnect automatically when the user changes the window size or leaves full screen.