Here are the steps to follow to make Opus always open the Tree, Find or Viewer panes with a particular size or orientation:
[ul][li]Open a new lister.[/li]
[li]Turn on the panes you want to adjust and set them to your liking.[/li]
[li]Select Settings -> Save as Default Lister.[/li][/ul]Opus will remember where the panes were and open them again with the same settings.
This will also save the current window size and directory as the defaults for new listers (assuming you have not changed Opus to open a particular Layout instead of the Default Lister).
The window position is also saved but you can, if you want, set new windows to always open under the mouse pointer via Preferences - Listers - Options.