Is there a preference setting I can adjust so that new, or existing, files are opened at full size rather than in a reduced format requiring re-sizing?
With “full size”, do you mean Zoom 100% or document window maximized?
Zoom level is saved with each document. New window size may be operating system dependent.
Can you share your operating system and LibreOffice version? To do that, choose menu Help - About LibreOffice and copy from there. Press edit below your question to add more information.
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If you mean the programs window size, i dont think that can be controlled by LO itself, but rather by the “window manager” of the operating system. As far as i can remember Windows should remember the Last Windows Size of a Program when it was closed … But my knowledge might be depricated here.