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.