Settings for fileuploads to Plesk backup repository can be changed in the admin php.ini file here.
The settings of php and could me adjusted by modifying the "%plesk_dir%\admin\php.ini" file.
To raise the time allowed to send a file into the repository
default_socket_timeout =<some digits> , where <some digits> is the time in seconds.
To raise the size of a file allowed to send a file into the repository
upload_max_filesize =<somedigits> , where <somedigits> is the limit for uploaded files in bytes