The last couple of posts are digging more into the larger (but still much smaller than Starfleet as a whole) force that started forming in the early days of the Federation through to roughly 2300 when it was substantially gutted either directly or indirectly as a result of the First Khitomer Accords.
Yeah, I think we've done the smaller force to death.
The larger force can be fun now. We can quibble over the specifics of just how large... yeah certainly not as large of an organization as the fleet, but I think this force can be quite large.
I've been doing a bit of research and even the largest of the fanon/licensed media transports of the era appear to top out at 2-3000 passengers for extended periods of time, and the majority of the (semi-)canon designs that could keep pace with Connies and the like are only good for a few hundred.
Which makes me think that even the larger force would typically go at least one size "smaller" than IRL USMC MEUs (150-330 for your "rapid reaction force" under a Major; then ~ 1,100 for your "intermediate force" under a (Lt) Colonel and 2,000 to 4,000 for your "main force" for short to medium term peacetime operations.
I'm not really sure the carrying capacity is really much of an issue. A Constitution-Class is still an incredibly large ship, it just has alot more space dedicated to other things. A purpose built transport should easily be able to carry thousands.
I'm also considering whether the "Marines" during this era should be the "specialist landing force" of a larger Federation Active Force/Reserve Force structure that otherwise more closely resembles the army, and who tend to take over once there is a need to employ larger formations or even whole commands for extended planetary conflicts, including but not limited to during actual war?
I'm still not huge on the idea of a Federation Army per say, but I do think ESPECIALLY in this era there would be something like a member world unified command for their forces? It might be somewhat splitting hairs as it essentially ends up with the same function, but I like to think that Starfleet encompasses the federal-level forces directly under the command of the Federation, while the rest, Reserve Force whatever you want to call it, is entirely composed of member world forces with an apparatus in place to ensure they can work together easily. I keep going back to the NATO comparison, although a structure more akin to the National Guard makes sense too.