Manual:$wgMaxImageArea/en

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 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.