There's no way to remove spaces from the indices, because the internals of Confluence relies on the index to provide a lot of functions. It's not just search. If you did stop indexing your Sandbox space, it would become utterly useless as half your macros and most of the navigation would stop working. You'd end up with a pile of unsorted, unsearchable pages with broken stuff all over them.
If you need a Sandbox, set up a dev/test server and use a developer licence for it.
Sizing a Confluence is not a simple formula. There's some very rough calculations you can do with total spaces and numbers of pages, but the main thing that feeds into the specification is "how are you using it". Hundreds of users who read it, but only a handful who will write? Hundreds of users who have woven it into their daily life so will be updating it many times a day? Do you tend towards large pages with lots of detail, or do you break them up into lots of little highly focussed pages? IO may be impacted by whether you use a lot of attachments or imports. And so-on.
For the rough guide which will give you a good starting point, start at https://confluence.atlassian.com/doc/server-hardware-requirements-guide-30736403.html
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