I don't think that a 9 inch reader is needed for screenplays. I'm currently using a 6 inch device to read plenty of pdfs, both fiction and academic, without much difficulty. If your device is good at handling pdfs, I believe you're fine as long it's simple text. Issues arise only with unusual formatting, small fonts, and, mostly, graphs.
But mind you, pdfs scanned as images are awful for e-readers. What I suggest applies to text pdfs only; if you have many image pdfs you need a 9 inch screen (and even this might not be big enough).