How to find disk space usage of logs returned by query

I am attempting to determine storage requirements for logging various systems. It's convenient when all logs for the system in question are going into one index e.g. all firewall logs going into a firewall index, I can just look at the daily index size and say firewall logs are ~x GB/day.

What I have not figured out is how to tell e.g. daily size of linux system logs sent to a shared index (e.g. shared with application logs).

e.g. questions like average daily disk usage within elasticsearch per host of /var/log/messages and /var/log/secure

Is there some way to query something like that, or would you need to run a query, count the documents and multiply by some "guesstimated average size of log message" factor?

I do not have the entire fleet going in, just some subset. We are negotiating with elastic currently and need to produce some sizing data for cost estimates.

You need to enable this plugin Mapper Size Plugin | Elasticsearch Plugins and Integrations [7.13] | Elastic.