Preview is Not Remembering Window Sizes and Positions on Startup (after Update to High Sierra) Since I updated to High Sierra the Preview app no longer remembers the sizes and positions of its open windows. Instead it just opens all windows on the same desktop with standardised size and position when the computer starts up.

How do I fix this?

(Side note: After updating the Preview app also didn't remember the last viewed page of PDF-files, but I was able to fix this by deleting the ~/Library/Containers/com.apple.Preview folder. However, the window size and position issue persists.)
Preview does remember the position of windows containing images (JPG, PNG, etc) but for some reason does not restore the position of PDF windows. I suspect this is a bug.

The saved state for Preview is located at: ~/Library/Containers/com.apple.Preview/Data/Library/Saved Application State/com.apple.Preview.savedState

Meanwhile, you can use an app like Stay to remember and automatically reset the position of any window. It’s a bit heavy handed for this use case but it would let you carry on until the bug is fixed.

You might also look at window managers such as Tiles or Rectangle that allow easy manipulation of windows.