I was wondering if it would be possible to confirm what the size of VII Corps was when the invasion of the Magistracy of Canopus began was?
The detail provided on page 95 of the pdf edition of Historical: Reunification War states that the makeup of VII Corps was three divisions (19th, 20th, 21st Royal) plus four independent BattleMech regiments. It then gives the force strength of the Corps as 31 BattleMech regiments, 17 Armor regiments, 30 Infantry regiments and 12 AeroSpace regiments.
In comparison to the other SLDF corps, that's absolutely huge. Page 65 and page 107 indicate that the corps involved in the Taurian and Outworlds Alliance campaigns were each formed of 9 BattleMech regiments, 9 Armor regiments, 9 Infantry regiments and 3 AeroSpace regiments - it's only VII Corps and the two corps involved in the Rim Worlds campaign that differ from this pattern.
Whilst I'm inclined to say that page 94 is in error, the description of the campaign for Cranston on page 98 states that:
The nine 'Mech regimentsof the Fifty-eighth Brigade expected a hard fight
- and that doesn't include the FWLM auxiliary forces, because it later states that there were
six regiments continuing to grind down the Cranston garrison while the remaining three regiments (together with a FWLM auxiliary regiment) prevented the Restitution forces from linking up.
The Commanding Officers of the various Brigades mentioned in the Magistracy of Canopus chapter are also all variously described as "General" so and so, in contrast to the Outworlds Alliance chapter, where the Brigade commanders are described a "Lieutenant General" so and so, which again infers that the VII Corps was atypical.