Are you using the direct MySQL connector (database type set to "mysql" or "mysql4" in config.php), or ODBC? The MySQL connector is controlled by php.ini, but there is no limit that can be set in there on post size.
However, the web server can have a limit on the maximum size of data posted via the POST method. For many services, this is 8 megabytes, but some hosts set it quite small. If you create a file containing the following and upload it as "phpinfo.php" to your website, it will tell you what you want to know:
You want to run that in your browser, and search for the value next to "post_max_size".