300-500 seems more than enough. A guild master should know who is in their guild. A smaller size will encourage removing inactive players in favor of active ppl. An auto remove after 90 days to save time going through the list.

Other games I play have caps. Auto replace inactive guild master to next high ranking player.
I’ve been in 1k+ member guilds and 200 member guilds. All had about the same amount of ppl online at any moment... 20-50 ppl at most.

One guild had 100+ afk officers...