Manual:$wgMaxUploadSize

Details
Maximum size of uploaded files, in bytes.

This is the maximum size of a single file upload (not the total of all uploaded files). It 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'. The value is the maximum size of the file in bytes.

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

This option currently has no effect on the normal upload feature for MediaWiki 1.17 and lower! If you've enabled $wgAllowCopyUploads, this setting does limit the size of files copied from a remote URL as expected. When uploading files from your local machine, this setting currently has no effect (for at least versions 1.9.3 through 1.12).

The limits for file uploads are controlled by PHP configuration directives. To change the limits for file uploads, you need to modify php.ini, see the upload_max_filesize and post_max_size PHP configuration options as well as Manual:Configuring_file_uploads for details. By default, PHP does not allow uploaded files to be more than 2 megabytes.