If I have 40 men in the majors thanks to promotion in a theme league do only 25 of them dress for any game? If so, how do I determine which ones?

I don't understand the question.

SLB rules allow only 25 players on the major league roster at any time. It is not possible to ever have more than 25 men in the majors.

https://www.whatifsports.com/mlb-l/playerstats.asp?ltid=2&teamid=1811592

This is a more than 25 man roster in a TOC so it’s clearly possible.

I have played in over 440 TOC and don't remember ever seeing anything like that.

I've played in leagues with active rosters much larger than 25. I don't know the mechanics of doing it since I didn't commish those leagues. Somehow, commishes can promote AAA players to the active roster above the limit of 25 players. I think Grizzly ran some $255M leagues like that. If he sees this maybe he can respond. It's possible, though.

Yeah, the maximum on a major league roster is 40 in theme leagues where the commissioner promotes manually for each team. This also has potential to ruin the $255 Million TOC now that rosters don't shrink to 25 at the roster deadline and you can carry 40 people into the TOC.

Like mensu, I have never seen this before, despite playing thousands of teams over 20 years on this site. So, I learned something today.

My assumption would be that if a player is on your ML roster, then he is eligible to be used in a game. I cannot conceive that somehow the SIM is programmed to choose a subset of the active players as eligible for any single game.