Space Marine Legions - Notes on Size: The size of a Space Marine Legion has been a contentious issue over the years, with various numbers mentioned in multiple sources intended to reflect the notional size of a Space Marine Legion.This matter is further complicated by the fact that the size of the different Legions may vary based on source material or an author's lack of background information, which can result in conflicting numbers.In general, the canon for the size of the Space Marine Legions has been set at numbers in the low hundreds of thousands, using the work of Alan Bligh in The Horus Heresy series for Forge World as the definitive source.Games Workshop Lead Games Designer and author Andy Chambers addressed this issue about pre-Heresy organisation in White Dwarf 272, "GW Mailbox," stating, "There really isn't any information on pre-Heresy organisation. We work on the principle that Space Marines started out in Legions approximately 10,000 strong (or more, depending on the specific Legion), which were broken down into Chapter-sized Great Companies rather like the Space Wolves (who are renowned for not adopting the Codex Astartes alteration made by Guilliman post-Heresy and who thus probably follow the pre-Heresy organisation more closely)."Note 1: In the novel Dark Creed by Anthony Reynolds, before the invasion of the Boros Gate by the Word Bearers Traitor Legion in the 41st Millennium, it is mentioned that the Black Legion's strength is unparalleled, and that they easily outnumber the Word Bearers by almost 10-to-1. This means that if the Word Bearers had somehow managed to maintain their ranks around 100,000 Chaos Space Marines, this would imply that the Black Legion has vastly increased its overall size since the Horus Heresy, with a primacy of around 1 million Traitor Legionaries.Note 2: In the novella The Crimson Fist by John French, it is mentioned that the Imperial Fists Retribution Fleet numbers around 20,000 Legionaries, which constitutes one-fifth of the entirety of the VII Legion. This makes their overall number stand at around 100,000 Astartes.Note 3: In the novel A Thousand Sons by Graham McNeill, it is stated at the conclusion of the Battle of Prospero that First Captain Ahriman counts around 1,242 Legionaries present during this conflict, and states that this constitutes approximately one-tenth of the XV Legion, which would place the Thousands Sons around 10,000 Astartes at their prime. But according to the information within The Horus Heresy - Book Seven: Inferno, released in February 2017, the estimated strength of the Thousand Sons stood at approximately 80,000-90,000 Legionaries, with 62,000 being present on Prospero. This source also goes on to state that some of the detachments fighting in the Great Crusade elsewhere in the Imperium were to be destroyed by their allies before news of Prospero's destruction reached them: 2,000 under Sentor Rahme of the Ninth Fellowship were slaughtered in their encampments by the Ultramarines attached to the same force, and 300 Veterans of the Seventh Fellowship were abandoned to their deaths during the assault on Maktor VIII by the Imperial Fists to name but two of the more prominent cases. Of all the main contingents of the Thousand Sons abroad in the galaxy, only two were known to have escaped Imperial justice, the majority of the Fourth Fellowship, some 5,000 Astartes who fled their Great Crusade assignment after hearing of the attack on Prospero, and a force, perhaps of several thousand, of the Sixth Fellowship present on the Forge World of Zhao-Arkhad as part of the agreement between the XV Legion and that distant and enigmatic Mechanicum forge. All of this information has obviously been retconned from the original numbers of the XV Legion presented in the novel.