Not sure if this is accurate, but this is what I go by in my universe. Excerpted from The Imperial Sourcebook (Star Wars West End Games)
Squad (9 people)
7 troops
1 Corporal
1 Sergeant
Platoon (38 people)
4 Squads
Lieutenant (Platoon Commander)
Sergeant Major (second-in-command)
Company (153 people)
4 Platoons
Captain (Company Commander)
Battalion (613 people)
4 Companies
Major (Battalion Commander)
Regiment (2453 people)
4 Battalions
Lieutenant Colonel (Regimental Commander)
Battlegroup (9813 people)
4 Regiments
Colonel (Battlegroup Commander)
Army (39253 people)
4 Battlegroups
General (Army Commander)
There's also a Systems Army, and a Sector Army, but it's also covered by Star Wars type characters and I'm changing it for My Traveller Universe. I'd probably say that a Systems Army is run by a Baron or Marquis and has 1 to 4 Armies in it (as much as they can muster). I'd throw in a Subsector Army run by a Count (4 Systems Armies), and a Sector Army run by a Duke (4 Subsector Armies). Plus a Domain Army run by an ArchDuke, added in (4 Sector Armies or as many as possible). The Armies can rotate at each quarter of the year. Example: Subsector Armies of Chronor, Jewell, Regina, and Aramis of the Spinward Marches would probably be on call for the Sector Army in the first three months of the year and rotate out for the other nine months on Sector Inactive (called if needed to Sector duty)
Later,
Scout