When starting the framework versions of applications (e.g. Dolphin, Kate and konversation), they always start with the default size of the window. It doesn't matter if the window was resized the last time the application was used, a new start of the application will return to the default size.  

As that it concerns multiple applications, I assume that the issue is not caused by the application itself but more in the underlying layers (Plasmashell, KF5 or Qt5).