tmux popups can be sized how you like based on the % width of the terminal on creation by using the flags (h, w, x, y) for height, width, and position.
# normal popup
tmux popup figlet "Hello"
# fullscreen popup
tmux popup -h 100% -w 100% figlet "Hello"
# 75% centered popup
tmux popup -h 100% -w 75% figlet "Hello"
# 75% popup on left side
tmux popup -h 100% -w 75% -x 0% figlet "Hello"
example running these commands