Battlegroup Overlord can be played from platoon per side up to battalion per side, the unit of maneuver is the squad or individual vehicle.
Piers and Warwick (the writers) mostly play Battlegroup in 20mm, I play in 15mm because I have Flames of War forces and 15mm terrain. Ground scale for either is roughly 10"=100yd/m
The Battlegroup army building system requires you to take an HQ unit and a certain minimum number of infantry squads or platoons (there is also a maximum number of infantry platoons in the larger games). You select forces from the 'Forward Headquarters,' 'Infantry,' 'Tank Units,' and 'Artillery Units' groups.
Each group then allows you to take different support choices.
Each Forward HQ unit (includes more than just the HQ, there's also comms and such) allows you to take one support choice from either Logistics Support or Additional Fire Support.
Each Infantry squad allows you to take one support unit from Reconnaissance, Engineers, or Specialist units, and taking a whole platoon allows you to take 4-6 (limited by the specific army list) units instead of just one.
Each Tank unit allows you to take one support unit from Reconnaissance, Engineers, Logistics, or Specialist units, and taking a tank platoon allows you to take 3 support choices instead of just one.
Each Artillery unit allows you to take a support unit from Additional Fire Support. (Artillery is things like Priests or M2A1 105mm guns, off-table light cruisers or destroyers, or Shore Fire Control Parties or AOPs).
You can only take defenses if you are the defender in an Attack/defense mission, and the bunkers, etc do NOT grant you access to any support units.
Reconnaissance support units are infantry squad patrols, or jeeps/armored cars, and snipers (but aren't available to the Amphibious Assault lists).
Logistics Support units are things like a supply column or medics/stretcher parties.
Engineer Support units are things like combat engineers, armored bulldozers, recovery vehicles, etc.
Specialist Support units for the Amphibious Assault lists are Support boat teams (officer, rifle squad, Mortar team, MG team, demolition team), additional landing craft, or the fire support landing craft. In both the US and British Amphibious Assault Battlegroup army lists, Landing Craft Gun, Landing Craft Rocket, and Landing Craft Flak are Specialist Support units, and you can only take one of each.
The Additional Fire Support units are things like high-priority fire requests, registered target points, timed barrages in various calibers (6" to 14"), off-table artillery in various calibers (3.5" to 14"), timed air strikes, and close air support.
The LCRs have a nasty rule called "Drenching Fire", which allows them to roll a Heavy HE Area Fire attack (very likely to pin, 1-in-6 chance to cause casualties) against everything within 30" of the center of the beach. It also has a 1-in-6 chance of removing barbed wire sections and minefields.
I really prefer the Battlegroup rules over Flames of War, the additional comms/whatever units and the way the army list works is much better.