You could edit php.ini IF you are managing the server by yourself. BUT if you are managing the server you would know anyway. So if it is a managed server by you i suggest you learn more about managing a server (or VPS).
Genarally speaking these are the values that has to do with uploading or posting in php.ini:
post_max_size
upload_max_filesize
Now beaware that changing values in php.ini settings affects all domains on a server so you must know and understand what exactly you change.
IMHO, i don’t think that such a post is limited by php.ini but i can’t tell you for sure before i see it. I would first made a few test, i.e. learn about the values of php from phpinfo(), try a shorter post to find the limit (if there is one) and then decide IF there where values in php.ini that preventing me to post.