RDR2 (2018) is the natural comparison for any new Rockstar open world. It set a new fidelity bar, a new wildlife bar, a new density bar for a non-urban world. GTA 6 will be measured against it whether the comparison is fair or not.
We work through where the comparison holds and where it doesn't. For the broader size question, see the map size piece.
Where RDR2 actually set the bar
Wildlife - extensive ecosystem, behavioural AI, predator/prey simulation. NPC density - small towns felt populated. Environmental fidelity - weather, season, terrain detail. Audio occlusion - the way sound moved through the world.
Some of these set bars Rockstar can clear; some are RDR2-specific because of the genre. Open-world wildlife in a 19th-century western is a different problem to open-world wildlife in 2020s Florida.
What GTA 6 has to do differently
Density of urban content - RDR2 had towns; GTA 6 has Vice City. The challenge is different. RDR2 didn't need to handle 1,000 NPCs in a downtown crowd. GTA 6 does.
Pace. RDR2 was deliberately slow; GTA 6 will not be. The physics, vehicle handling, and minute-to-minute mission pace will all be very different.
Where the comparison fails
Total map size. RDR2's playable area is large; GTA 6's might be larger or smaller. The comparison doesn't tell you about the experience because the games do different things.
Wildlife depth. GTA 6 will have wildlife - alligators are visible. Whether GTA 6 has RDR2-tier ecosystem AI is a separate question and probably the answer is no - GTA's wildlife has historically been lighter than RDR's, by design.
What we can fairly say
GTA 6 will be denser than GTA 5 and almost certainly denser than RDR2's settled areas. GTA 6 will be smaller in total physical map than RDR2's measured region. GTA 6's wildlife will be less ecology-deep than RDR2's. GTA 6 will be more pedestrian-AI-heavy than anything Rockstar has shipped.
All of this is informed expectation, not promise.