(Credits to Virtuoel)
If you've got an unplayable scale, theres a command-based workaround in Pehkui 2.1.0 and newer that'll reset all your scales back to their default values.

/scale debug delete_scale_data username YourUsername
If you're in singleplayer, enter a different world where you can use commands and run the above command with your username, then rejoin the world where you had the bad scale.

If you're on a server, have someone that's able to use commands either run it ingame or through the server console, then log back into the server.