This is probably an issue with your php.ini settings you would have to get updated on the server.
PHP has a couple of settings that could limit the form size:
Most likely it is the post_max_size that you are running up against. You might be able to change it in your htaccess file:
php_value post_max_size 10KB
Check your phpinfo to see which one is the likely problem and talk to your hosting provider about changing the value if the .htaccess entry doesn't work.