I've seen some games that have little arrows in the bottom right of the chat and when you drag it the chat gets resized. I have recently tried this but I was wondering if I could get some help. When I made my script there were a ton of errors that I wouldn't even be able to fit in this question. This is not a script request, I'm just wondering how the chat would get resized.
This is usually done with editing the chat scripts.
game.Chat
childrengame.Chat
ChatSettings
under game.Chat.ClientChatModules.ChatSettings
Line 15 specifically module.WindowResizable = false
Doing so will enable the default chat window to be resizable.