The buyer's order quantity for a given download item would multiply the number of attempts you allow for them to complete a download of that product successfully -- e.g., if you allow 5 attempts within 120 hours to complete a product's download, and a buyer orders 10 of that product, they will be issued just 1 download link for that product which allows 50 (=5 x 10) attempts within 120 hours (note that your time limit in hours will remain the same).
9 attempts should be plenty for anyone to successfully download one file to completion. You wouldn't want unlimited attempts, because that would defeat the whole purpose of issuing each buyer a unique link that expires -- the idea being that even if they share their link with others, it could never be valid forever if the link URL happens to get "out in the wild". Is there some reason you would need to allow more than 9 attempts to complete a single download?
We are redesigning our entire Seller Admin in HTML/CSS/JavaScript to replace the Flash interface we have now, so that should eliminate the need to have every screen fit within the same area, which may be contributing to some screens appearing a bit cramped/cluttered now. You are correct that many of the text-entry fields are too small for composing lengthy messages, so it's better to compose in Notepad or other text-editor and then copy-paste into the field.
However, regarding thank-you emails in general, it's a good idea to keep those as short and to-the-point as possible, as the more words and URLs you add to your thank-you email, the greater the chances it could get erroneously flagged as spam by the buyer's email provider. You can put whatever instructions and promotional text you wish in your thank-you pages without running that risk of being seen as spam, so it's best to leave the thank-you email as a simple "Thanks for purchasing Product X, which you can download here:" (which is basically what our default thank-you email says anyway).
