You can run Stronghold 3 in a window to do this. Enter the folder where Steam is, such as C:\Program Files\Steam\steamapps\common\stronghold3\bin\win32 _release. Within this folder there is a config.xml file. If you edit the config file with notepad and edit then add windowed=1 <resolution width=1024 height=768 windowed=1/>

To change the Windowed size change the resolution while in-game.