Please don't byte me if I'm wrong, but if I'm correct it's like this.
Copy the php_settings.json from /usr/local/directadmin/data/templates to /usr/local/directadmin/data/templates/custom and if the custom directory does not exisit, create it.
Then in the custom directory, edit this php_settins.json file and add the values you want to post_max_size and max_upload_size in there.
Here is an example I found just now.
Is it possible to change a given range to a list of values? Example: change range from "max_input_vars" : { "default" : 10000, "type": "int", "range" : [ 1, 100000 ] }, to a list of values: "max_input_vars" : {...
forum.directadmin.com
This is for the max_vars but this way you can also change other settings you want.