To maximise the upload size specifically, create a file at .ebextensions/nginx/conf.d/proxy.conf set the max body size to whichever you prefer
Create a change in your config file inside of .ebextensions to supplenment the nginx file. With this change, you can have a bigger post body size.
Inside the .ebextensions directory, create a 1file.config with the following contents:
This will generate the proxy.conf file inside the given directory which contains the single line client_max_body_size 20M.
Check out this link to learn more about Nginx configuration: