The HALO games focus mainly on the Marines in the UNSC Defense Force, and the games often depicts formations that are broken up due to combat and and battlefield losses, especially with the large amount of losses in canon. Its unsurprising that you see so few full strength units in the games, but I want to put together the Army and Marine Corps proper battle formations (as reflected by modern US Army and USMC formations with some wiggle room taken by myself).
UNSC Army (Infantry) Human-Covenant War Era
Led by a Sergeant (E-5) or Staff Sergeant (E-6)
Platoon (Smallest unit capable of effectively maneuvering and conducting combat operations):
3 Squads (30 men)
Led by a 2nd/1st Lieutenant (O-1/O-2) assisted by a Sergeant First Class (E-7) as Platoon Sergeant
3 Platoons (90 men, plus an Headquarters and possibly Support units)
Led by a Captain (O-3) assisted by a First Sergeant (E-8) as the principal NCO and an HQ unit.
Battalion (considered tactically and administratively self sufficient):
3-5 Companies (270-450 soldiers, plus HQ and support staff)
Led by a Lieutenant Colonel (o-5) assisted by a Command Sergeant Major (E-9) as the principal NCO plus his HQ staff.
Company level combat units will be issued MA5 variant weapons for all mainline troops, plus NCOs of the rank of Sergeant (E-5) or higher will be issued a sidearm. HQ units of Battalion level will only be issued sidearms, but may acquire a rifle or SMG if they so choose.
A Squad will also be issued a small number of 40mm grenade launchers (for fireteam and squad leaders) and possibly a shotgun (issued by mission demand).
MMG (Medium Machinegun) units are squads of 3-4 men issued with an M247 GPMG (gunner also issued a sidearm) as an attachment to a Platoon sized unit or higher. Similarly a MAT (Medium Anti-Tank) team is 2-3 men issued one M41 rocket launcher (gunner also issued a sidearm) on a Platoon level or higher.
A Sniper team is a 2 man team issued on a Battalion level, or higher, that is armed with one SRS99 rifle for the Sniper and the Spotter is issued with a M392 rifle. The team are each issued with sidearms. A Scout ream is also a 3-5 man unit deployed on a Company level or higher armed with M7 SMGs, MA5 variants, or M392 DMRs.
Vehicles and crews (usually M12 Warthogs and variants, M274 Mongoose, AV-14 Hornet, D77 Pelicans, or UH-144 Falcons are dispersed from Company level by the discretion of Battalion HQ. The crewmen are armed lightly, usually with an SMG or MA5 variant.
Medics are deployed on a Squad, Platoon, and Company level. This means each squad has its own medic, with another at the Platoon level, and another at the Company level.
UNSC Marine Corps
4 Marines (team leader, 3 marines)
3 Teams (12 Marines) plus Squad Leader and Medic
3 Squads (44 Marines) plus Platoon Leader, Platoon Sergeant, Platoon Medic
5 Platoons (3 Line, 1 Weapons, 1 HQ "Platoon") (172+ Marines)
5 Companies (3 Line, 1 Weapons, 1 HQ and Service) (688+ Marines)
The Marine Corps is closely the same as the Army with higher numbers per squad and relevant infantry/fighting units, and weapons for standard units. NCOs are armed with MA5 variants and a sidearm while Marines are armed with BR55 variants.
MMG, MAT, Vehicle Crews, Snipers, and Scouts function and are armed the same way as their Army counter-parts. Corpsmen still supplied by the Navy.