Official size
Rockstar's installation requirements page says: "Grand Theft Auto VI will require up to 212 GB of available space depending on your device." PlayStation's page tells players to clear a minimum of 212GB. That is the number to plan around.

PS5 vs Xbox
| Platform | Official guidance | Reported figure |
|---|---|---|
| PS5 | Minimum 212 GB | About 209.75 GB download / 212 GB installed, reported from early preload screens |
| Xbox Series X|S | Up to 212 GB depending on device | About 209.9 GB on console, from store-listing reports |
The reported figures are not Rockstar numbers. Use 212 GB as the safe target on either console.
Standard vs Ultimate size
No separate install size has been announced for either edition. Both include the same base game; the Ultimate Edition adds in-game content. A day-one patch size is also unconfirmed.
How to make room
On PS5, delete or move games to an M.2 SSD; USB extended storage cannot run PS5 games directly. On Xbox Series X|S, use the internal SSD or a Storage Expansion Card. Leave some free space beyond 212 GB for saves, captures and a possible patch.