Mac app opens at the wrong size or position? How to fix it

macOS leaves window sizes to each app. Most remember the size and place of the last window you closed and reopen there. When that memory points at a display you no longer have, or a size from another screen, windows open in odd places.

Fixes, quickest first

1. Resize it, then quit with it open. Set the window the way you want, then quit the app with ⌘ Q instead of closing the window first. Most apps save the size on quit.
2. Use Fill. On macOS Sequoia and later, Window, Fill (or ⌃ Fn F) makes the window fill the screen without going full screen. Then quit with ⌘ Q.
3. Wrong display: open the app's Dock menu (right click its icon), Options, and choose the desktop or display to assign it to.
4. Off screen: Window, Move to the display you are on, or see bringing a window back on screen.
5. Still wrong: some apps keep a saved state that goes bad. Quit the app, hold ⇧ while opening it to start without restoring its old windows, then size the window again.

Make every window open in a set place

Fixing one app at a time does not stop the next one. A window manager decides the size for you, every time.

ClutterFreeMac gives each new window a column on the strip at your default width, so a window never opens tiny or half off the screen. In Settings, Apps, you can set an app to open full width, always float and keep its own size, or keep the start or end of the strip, so it always lands in the same place.