Manual:$wgMaxImageArea

Details
This value specifies the maximum number of pixels a source image can have if it is to be scaled down by a scaler that requires the full source image to be decompressed and stored in decompressed form, before the thumbnail is generated.

This provides a limit on memory usage for the decompression side of the image scaler.

The limit is used when scaling images with any of the built-in image scalers, such as ImageMagick or GD.

It is ignored for JPEGs with ImageMagick, and also when using the vips>Special:MyLanguage/Extension:VipsScaler|VipsScaler extension as these situations do not cause excess memory usage.

Default is 50 MB if decompressed to RGBA form, which corresponds to 12.5 million (1.25e7) pixels or 3500×3500.

To be clear, this only prevents scaling of large images. Users are still allowed to upload large files.