You can either download the script via the releases section, clone the repository, copy-paste the code like a degenerate milksop, or — if you're on Mac — install via Homebrew:

$ brew tap solidiquis/tap
$ brew install alacritty_font_size

I would most definitely recommend creating an alias for usage in the command line as the name is quite long.

• <Up> | k to increment font-size.
• <Down> | j to decrement font-size.

Exit with Ctrl-c.