TL;DR: No.
If you have a program that depends on terminal size, you are likely Doing It Wrong. The whole point of a terminal is that anyone can run any program. If you need a minimum window size, you should probably consider a GUI.
Tmux has setw force-width <value> and setw force-height <value> but they are only able to change window heights in the allowed range which is dictated by the smallest session size that is connected. If the values are larger than can be used, it will set the dimension to be the max that it could be, given all connected session constraints.
Tmux has resize-pane -x <value> -y <value> but they cannot exceed the size of the window and will not truncate if there are not other panes below to bring up. If the values are larger than can be used, it will set the pane dimension to be the max that it could be.
Tmux has setw -g aggressive-resize on which can be used in combination with tmux new-session -t <base session> -s <new session> so that if multiple connections exist to one session and they are viewing different windows in that session, the connections will only enforce sizing on the windows they are viewing, not on all windows in that session.
The only way another person can see your tmux sessions is if they are logged in as the same user (unless you've actively done configuration to change this). If other people are logging in with your username, you are again Doing It Wrong. Tmux is designed to explicitly allow a bunch of connections to a session; the whole point of tmux is to allow a user to connect to sessions from multiple machines even if other connections are still going or haven't timed out yet.
You could write a cron script that uses tmux's detach-client option that you referenced in your question to disconnect certain clients, but it wouldn't prevent the window size from at least temporarily changing.
