This option will enable you to specify the chunk size that the multiple upload feature will use. Chunking enables you to upload very large files regardless of post/upload restrictions set by the hosting provider. Notice that you can use mb, kb suffixes.
Example of usage in the config.php file
$mcImageManagerConfig['upload.chunk_size'] = '3mb';
Example of usage in the Web.Config file
<add key="upload.chunk_size" value="3mb" />
Example of usage in a mc_access file