Dishonored 2 requires a 9 GB day one patch on PlayStation 4.
The file size was first reported by Reddit user LegalTRR, but GameSpot has independently confirmed that the PS4 version automatically triggers a 9.1 GB patch.
GameSpot can confirm the Xbox One version also prompts a 9.0 GB patch download.
While it is possible to play the game without installing the patch, doing so will sign you out from PSN or Xbox Live. Dishonored 2's publisher Bethesda recommends you do install the update.
The benefits for updating, as listed by LegalTRR, include improved loading times, texture streaming, audio mixing, frame rate, and shadow quality, as well as numerous bug fixes.
Dishonored 2 releases November 11 on PS4, Xbox One, and PC.
Bethesda recently revealed the minimum settings you'll need to run Dishonored 2 on PC--they seem fairly reasonable, though recommended settings are a little more demanding.