I found a solution.
I added a command in the properties section of a shortcut for the totala.exe. The command has to be added in the adress for the targeted *.exe of the shortcut(in this case totala.exe).
C:\CAVEDOG\TOTALA\TotalA.exe -d 1280 800
The added command is "- d 1280 800"
It enables window mode with the window size 1280x800 (I just used the size of my fullsreen size and it works)
Bye
EDIT: This only works for offline playing, because the game client(s) start TA directly over the totala.exe.