RESCENE will probably just do a two-day handball arena show at first.

Idol concerts, unless they are at the level of BIGBANG, generally have less demand than solo artists like Sung Si-kyung or Crush.
I expect the first concert to be in the first half of next year at the earliest. By then, to put it nicely, the craze will have settled, or to put it bluntly, the bubble will have burst. So, they will test the waters with a 2-3 day show at the Handball Arena for about 10,000–15,000 people, and if demand overflows, they might move to the Gymnastics Arena.
For girl groups, Blackpink is the only one that can go beyond Gocheok Sky Dome. Even aespa did two days at Gocheok, but they couldn't sell it out even with many Chinese fans attending.