RE: What is the max size of a string allowed inside a template?

The max string length in Self is 1k. If you need to return text greater than 1k, you need to use a Paragraph object.

var text = new Paragraph();
text.append(#sentence, "sentence 1.");
text.append(#sentence, "sentence 2.");
text.append(#sentence, "sentence 3.");
return text;