I can’t tell how much database size will be required: it all depends on your content and your indexing settings. There are too many variables. All I can say it probably takes 3-5 times the amount of space required by your wp_posts
database tables.
Relevanssi will have a separate index for each multisite site. For wp_1_posts
, the index will be in wp_1_relevanssi
and so on.
All changes will be indexed automatically. You only have to run the manual indexing once per site.
Relevanssi should have no effect on page performance outside search results pages. Search results pages may be slower than before, but the results will be better.
As for the excessive database space, define excessive. Some will think Relevanssi takes excessive amounts of space, some won’t.