Unfortunately there's not a lot to go off reliably with canon. The Empire has a 400m Frigate with a crew of 20,000. You're talking 100,000 personnel there as feasible, which I would agree is somewhat outlandish. Transport capacity is somewhat smaller, but if you imagine that training a competent spacer technician is any less arduous or expensive than training a soldier, well, that's a different discussion.
My main guideline has been that I recall it being mentioned (though I can't rightly remember where) that a Tier 2 Company can be generally assumed to be self-sufficient and stable, albeit potentially niche and otherwise not having a lot of impact on a galactic scale. To that end, I think a modern brigade-sized force is pretty reasonable, given that several thousand troops is practically a raindrop in the ocean compared to the armed might of even the smallest major faction (and larger minor factions) but is enough to support itself in, say, a minor conflict between to local planetary warlords (who can be assumed to field roughly nation-sized armies).
It's also worth noting that Shadowline only fields the aforementioned veteran troops (and necessary noncombatant support crew) and doesn't have an equivalent or larger force of 'grunts' to back them up. In a situation like that, where you have a large-scale PMC which does multiple operations (like garrisons, security, and combat operations) then I would agree that a full brigade of veterans is a bit much, but I'm also talking a company with total personnel under arms of around 10,000 or more.
Bear in mind that a Tier 5 company was just recently able to engage the navy of a major faction and walk away victorious. There's the human element to be factored in there as well but I think it gives a nice sense of scale.
My two cents, I suppose. Obviously the proper way to handle things would be to make the sub and hash it out with the Factory Judge.
