There are several sources that do give what seems their table of organization.
There's a Hammers TO&E in the 1979 book. It's one of the essays that separate the short stories.
I'll take a position and speculate that a regimental combat team is about as large as you can get independently, as there may be political and internal security reasons not to expand beyond that.
Political, security, and equipment issues. In the Hammers' case, Friesland raised and equipped the regiment to put down a colonial revolt. After successfully putting down the revolt, the regiment then mutinies when Friesland sends national forces to disarm and, presumably, kill them.
The idea that only governments can afford to raise and equipment certain forces is repeated throughout the Hammers series. The Hammers exist because Friesland initially equipped them and then keeps some sort of quasi-official, plausibly deniable relationship with the regiment even after the mutiny. Dutch is the unit's
lingua franca, many recruits still come from Friesland, and Hammer officers even attend military academies there.
In both the 1979 essays and other stories in the series several other units are said to have been raised, equipped, and hired out by national governments following the Hessian model. Even the Hammers themselves end up in that role after the Colonel becomes president of Friesland.
The Slammers are armour heavy, Falkenberg's Legion is primarily light infantry, but later revisions give it a more balanced force.
Falkenberg's merc unit initially exists solely through sleight of hand by the CoDo Navy. An entire regiment of marines and their officers are either demobbed or retired and their equipment "lost". The navy then uses the unit to tackle problems the Grand Senate won't allow actual CoDo forces to intervene in. Again, the mercs only exist at the size and capability they do because of official, if secret, support.
Condottieri, probably under a different title, will accept contracts for large scale deployments of disparate mercenary units, and try to create some cohesive field force out of them, whether brigade or close to divisional size. They'll be responsible to their paymaster for the behaviour of troops under their command, and to the troops for working conditions and wages.
You'll see hints and outright references to that in the Hammers series with the Slammers only sometimes acting as the coordinating body. In
Paying the Piper the war is slowly being lost because there isn't a coordinating authority.