Manual:$wgMaxShellFileSize

Shell and process control: $wgMaxShellFileSize
Maximum file size created by shell processes under linux, in KB.
Introduced in version:1.8.0 (r16665)
Removed in version:still in use
Allowed values:(integer)
Default value:102400

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Maximum file size created by shell processes under linux, in KB.

ImageMagick convert for example can be fairly hungry for scratch space.

See also edit