Via [url=https://steamcommunity.com/games/CSGO/announcements/detail/3800535912282829074]the Steam Community[/url]:

[quote=Valve][b]GAMEPLAY[/b]
- The visual and audio feedback from sub-tick input—such as movement acceleration and muzzle flashes—will now always render on the next frame
- Random numbers used for shooting spread are now synchronized between clients and servers by default. This makes tracers and decals match their server-authoritative trajectories more closely. Note: Spread has been synchronized on Valve official servers since 10/14
- Fixed a bug where pressing a movement key immediately after jumping resulted in an inconsistent jump height

[b]MATCHMAKING[/b]
- Parties of four are now able to queue for Premier mode
- Added vote-kick immunity for solo players who get matched on the same team with a party of four players
- Adjusted the algorithm for re-establishing CS Rating after a period of inactivity in Premier mode

[b]AUDIO[/b]
- Reduced the maximum audible distance of grenade bounce sounds
- Improved occlusion filter quality
- Addressed issue where sometimes headshot dink sounds playing as feedback for the attacker could be mistaken for incoming damage
- Lowered volume of sound for headshot damage with no armor from a victims perspective to bring it inline with volume of other headshot sounds
- Changed jump land sounds to have the same maximum audible distance as footstep sounds
- Changed volume falloff curve of jump landing sounds to better convey distance
- Improved molotov extinguish effects to sound better when multiple are playing at once
- Brought back spark sound feedback when bomb defuse is cut short due to player spinning
- Fixed issue where at very close proximity, a victims death groan could be mistaken as coming from the attacker
- Fixed a case where unsilenced m4a1-s did not have any distant gunfire sounds

[b]MAPS[/b]

[b][u]Mirage[/u][/b]
- Fixed clipping on bench in palace
- Lowered the height of box on A site to match CS:GO

[b]EFFECTS[/b]
- Improved performance of blood effects at close range
- Improved bomb light visibility from back of bomb
- Reduced muzzle flash during quickswitch from scoped rifles
- Reduced the strength of the kill streak HUD effect after five kills

[b]CSTV[/b]
- Added setting "tv_record_immediate" to control when CSTV demo file will start recording content. If the setting is enabled then content will start at the moment when tv_record is executed on the server. If the setting is disabled then content will start recording from tv_delay earlier
- CSTV demos can now be recorded without delayed CSTV spectating enabled. Requires setting "tv_record_immediate 1" and executing "tv_record" at the moment when recording should start

[b]MISC[/b]
- Adjusted wear values of some stickers to better match CS:GO
- Fixed a bug with workshop tools that would prevent light_environment from baking static shadows in some circumstances
- Improved quality of 'Performance' HDR rendering mode
- Enabling the upload of workshop maps for Wingman mode[/quote]

Rumor has it:

- Size is close to 325 MB (with Workshop Tools installed)