Poster size.txt

For movie posters in Plex, the standard aspect ratio is 1:1.5, or 2:3, meaning the width should be approximately 2/3 the height. A common size you'll find online is 680x1000 pixels, though this is slightly wider than the ideal 1:1.5 ratio. Aim for a minimum resolution of 600 pixels wide, with higher resolutions being better. 
Elaboration:
Aspect Ratio:
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Plex uses a 1:1.5 aspect ratio for movie posters, ensuring they are displayed correctly in different Plex clients and library views.
Minimum Size:
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A minimum resolution of 600 pixels wide is recommended for movie posters, but higher resolutions are always preferred for better image quality.
Example Size:
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Many online sources, like TheTVDB, use 680x1000 pixels for posters, which is close to the 1:1.5 ratio.
Background Art:
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For background art, also known as fan art, Plex uses a 16:9 aspect ratio, which is the standard for widescreen televisions.