This publication contains the options to customize the text.
local chatService = require(game.ServerScriptService:WaitForChild("ChatServiceRunner"):WaitForChild("ChatService"))
chatService.SpeakerAdded:Connect(function(playerName)
local speaker = chatService:GetSpeaker(playerName)
speaker:SetExtraData("Font", Enum.Font.FredokaOne)
speaker:SetExtraData("TextSize", 40)
end)