A simpler solution on recent versions of tmux (tested on 1.9) you can now do :
tmux detach -a
-a
is for all other client on this session except the current one
You can alias it in your .[bash|zsh]rc
alias takeover="tmux detach -a"
Workflow: You can connect to your session normally, and if you are bothered by another session that forced down your tmux window size you can simply call takeover
.