Manual:$wgMaxUploadSize

Details
This setting allows you to place a maximum size on each file that can be uploaded to your wiki.

The value(s) you specify refer to the maximum size, in bytes, of a single file upload (not the total of all uploaded files). The limit is applied at the time of upload, so lowering this value at a later date won't remove existing files that are now 'too large'. This limit only applies to chunked uploads and uploads without separate chunks might have a lower upload limit based on the PHP setting.

There are 1024 bytes in a kilobyte, and 1024 kilobytes in a megabyte, so  = 100 megabytes.

Specifying an integer
This limits the maximum size of uploaded files, in bytes, regardless of how they are uploaded.

Specifying an array
If you would like different maximum sizes for file uploads and upload-by-URL, you can set this variable to an array. For example:

This example will set the maximum for all uploads to 250 KB, except for upload-by-URL, which will have a maximum of 500 KB.

To enable upload-by-URL, see.