Is there any size limit to post a file using curl?

Upload limits are a security feature. Without them, a rogue program or attacker could feed your server with a continuous stream of data until your hard disk is full, thus rendering the whole server unusable.

From the security standpoint it isn't particularly useful to restrict outgoing data and, as far as I know, neither the Curl library nor PHP itself impose any limit.

Your symptoms suggest the problem is on the destination server. Since you appear to have access to it (you mention getting empty $_POST) I suggest you verify upload limits there. That's something you can do (and often change) yourself, you don't have to ask the server administrator. Main involved directives include:

• post_max_size
• upload_max_filesize
• max_file_uploads
• max_input_time

You can inspect them with phpinfo() or ini_get() and you can change them the usual way.