Raid and Dungeons Scaling Information

Raids: 10 & 25 man raid scale starts from 5 for 10, 10 for 25
and continue to scale per person, example 20 players would scale as 20 players
and full groups for Raids/dungeons will be as they are normally.
Raids before ICC 25+/RS are soloable/per person scaled and scaling harder with 5 and above players

Trial of the Crusader: 1+ for 10 Player, 3+ for 25 Player

Icecrown Citadel: 1+ for 10 Player, 10+ for 25 Player

Icecrown Citadel Heroic: 5+ for 10 Player, 10+ for 25 Player

Ruby Sanctum: 6-8+ for 10 Player, 10+ for 25 Player

Ruby Sanctum Heroic: 8+ for 10 Player, 10+ for 25 Player
VOA ICC Toravon Boss: 5+ for 10 Player, 10+ for 25 Player

BE AWARE: Due to boss mechanics and players skill levels some fights may require a different amount of players

SCALING

Raids before ICC 25+/RS are solo and group scaled meaning you can solo and be in small groups of 1 to 4 people but once the group goes above 5+ players it will scale normal and be a lot harder and like the real thing.

Some bosses are designed for more than 1 player so you will still need another player in some cases and some classes will be better than others at solo content.

SOLO RAIDS AND SCALE NORMAL ABOVE 5+ PLAYERS

Naxxramas
Obsidian Sanctum
Eye of Eternity
Ulduar (Exception Yogg-Saron/Thorim, not solo 2 bosses)
Onyxia
Vault of Archavon (Exception ICC Toravon, not a solo boss)
TOC normal
ICC 10 man normal only and doesn't scale harder with 5+ (depending a lot on gear)

DUNGEONS

Soloable
Scale from 1 to 5 Players

Mythic Dungeons Mythic Leaderboards

Timed dungeons with rewards and require 5 players (based on mythic dungeons on retail)
Another good way to farm current tier items!
Check the mythic vendor in-game