The only way I can see scale is when I look at /kill tags and see the option to kill only mobs with certain scales. I cannot change the scale of anything using the /scale command as it doesnt exist.
Not sure if this is an issue but I just want to be able to change the size in forge.

The command not existing makes me wonder if you're connected to a server that doesn't have the Forge version of Pehkui installed.
If it is installed, check the Pehkui config for your client (and the server config if multiplayer) and make sure enableCommands is set to true.

However, since you've not provided any information about your Forge version, installed mods, whether you're in singleplayer or multiplayer, etc., I don't know how accurate or not these guesses are.

If my above guesses are wrong, please create a new issue using the Bug Report template so I have more information to work with.

Closing due to inactivity / no response.