Old bubble (LegacyChatService)
New bubble (TextChatService)
Changing the corners of the bubble is simple:
chatService.BubbleChatConfiguration.UICorner.CornerRadius = UDim.new(0, 8)
But how do you change the size of the tail or the size of the bubble itself?
You can add a background image to the bubble, with the image being your own custom bubble size/shape.
https://create.roblox.com/docs/chat/bubble-chat
Edit: I actually don’t think this will affect much; you might have to make your own bubble chat system.
In short, there really isn’t much you can do, but a solution is fetching a bubblechat component in CoreGui, where you can edit it as you please, main issue being you will probably have to write a standalone system.
I tried that, but I wasn’t able to change the size of the ImageLabel itself or even touch the tail’s size at all.
Then thought about uploading an actual bubble, but eh… can’t quite scale the tail and bubble independently
Own system doesn’t even sound bad at that point 
How would I do that?
Whenever it comes to the CoreGui I’m never allowed to do/change anything. When showing it in Studio, I am able to view the contents of a bubble under ‘ExperienceChat’, but whenever I try to refer to ‘Caret’ (the tail) or such it doesn’t let me
