I would suggest using a user var named something like ZEN_MODE.

You can set it using something like:

printf "\033]1337;SetUserVar=%s=%s\007" ZEN_MODE `echo -n on | base64`

or turn it off:

printf "\033]1337;SetUserVar=%s=%s\007" ZEN_MODE `echo -n off | base64`

In your config, you can handle the zen mode changing using something like this:

local wezterm = require 'wezterm'

wezterm.on('user-var-changed', function(window, pane, name, value)
  local overrides = window:get_config_overrides() or {}
  if name == "ZEN_MODE" then
     if value == "on" then
       overrides.font_size = 18
     else
       overrides.font_size = nil
    end
  end
  window:set_config_overrides(overrides)  
end)

return {}

There are some caveats with this:

• the font size is scoped to the window, but a user-var is scoped to a pane. There can be multiple tabs and panes associated with a window, so you may wish to add more logic to reconcile what you want to happen when you're working with multiple panes/tabs.
• Cancelling zen mode requires sending a user var in the implementation above, which may result in some frustration if you don't have a convenient way to reset it. You may wish to add a key binding that clears the overrides.

An alternative implementation might be something like this, which allows you to use the existing/default CTRL-0 key assignment to reset the font scaling:

local wezterm = require 'wezterm'

wezterm.on('user-var-changed', function(window, pane, name, value)
  if name == "ZEN_MODE" then
     if value == "on" then
       -- Each IncreaseFontSize will increase by 10%, so if you want it 20% bigger,
       -- call this twice.
       window:perform_action(wezterm.action.IncreaseFontSize, pane)
    end
  end
end)

return {}

See: https://wezfurlong.org/wezterm/config/lua/window-events/user-var-changed.html