Is there a limit to how much text can be placed into a textbox?
At least 100,000 characters. Most likely a whole lot more.
If you have that much text, you may want to consider partitioning your texts into an array of strings so it is more manageable.
I’ve had a comma separated list of over 10,000 names in one text block for an experiment.
I’ve also had an entire technical glossary worth of definitions in 1 app variable. It’s really hard to work with and starts to lag the block editor but it works.