Hi Bert, 

thanks for your message.

This might depend on the server configuration. I can tell you how to increase this limit, but as it not strictly linked to the software and its functions, this goes beyond the assistance I can provide you with, as you will have to get in touch with the provider for any further inquiry related to this issue.
First of all, you should verify if you can find the file .htaccess. Once you find it, you should add the following code in it:

#set max post size
php_value post_max_size 20M

If you can't find it, you should create it containing the above mentioned instructions.

Unfortunately, I cannot help you further with this inquiry, as unfortunately the result of this procedure depends on the specific configuration of the server. If something cease to work, simply remove the instructions or file inserted to go back to the previous stage. I also suggest you to directly get in touch with your server provider, so to receive more instructions.

Thanks! Kind regards.