You need to find which configuration ini file has been loaded by PHP. The easiest way to do this is to run the phpinfo() function from the command line.
php -r "phpinfo(); " | grep -i "loaded configuration file"
You should see something like "Loaded Configuration File => /usr/local/etc/php/5.4/php.ini"
You should be able to open and edit the file. You might need sudo permissions depending on where that file lives.
If you can't execute that from the commad line you can always save an info.php file and run it from a web browser
Next look for the loaded configuration file path.
Once you've opened the file change these two directives (they are in different sections of the file).
upload_max_filesize = 2M post_max_size = 2M
Change the 2M to the size you want.