You could set the desired size of the window & save it as a layout (options -> lister layouts -> save this lister) & in -> preferences -> Opus start you could (for example) configure it to start that layout by double click on the Opus icon in the notification area.
You can find your old threads quickly by clicking the "Profile" button under any of your posts, then the "find all posts by Imageman" link on that page. Or you can use the Search page that's linked from the top of ever forum page and tell that to search by your user name.