A chat message on Twitch is limited to 500 characters. This same limitation is applied to redeems that requires the user to input text.

Let the streamer be allowed to choose a custom max limit on a reedem like this, which is useful for when you redeem a phrase to be read by the streamer. When a limit like this is set, a character counter can be displayed when typing.

Like for polls, there could possibly also be an option to be allowed to "purchase" additional blocks for points (done automatically). For example, say a phrase redeem costs 500 points, and you get a limit of 100 characters (5 pt/char). But for every additional 50 characters, the price increases by 100 points.

With the option to pay more for typing more, the price of the redeem would update automatically, and make it clear for the user how much they will pay for the current amount of text.