When I published the second edition of my novel, I created the files to upload to publishing sites (Amazon, IngramSpark). Once the epub and paperback were done, I went back to my manuscript and created a Large Print version.
I started with the files used for the standard text novel (12 pt), then I saved a new version of the files and expanded the text to 16pt for the Large Print. I checked the layout, chapter breaks, etc. to ensure the pages were ready to print (adjusting as needed). Since the novel then became thicker (larger text = more pages), I created a wider version of the cover and added “Large Print” to the cover art.
(Other things needed to be done, such as using a new ISBN, new barcode, and re-pricing the thicker book, etc.)
Now, I do have the advantage that I worked in publishing and used to help people typeset books, and can use my experience for my own books! This means that creating this new, additional Large Print version of each novel is using my own time. If you’re paying to have your book layout prepared, then you’d be paying for someone else’s time to essentially typeset another book.
Even though there are added costs and time, I believe it was worth it to make my book available in Large Print for readers who need (or prefer) easily-readable 16pt text. Having purchased Large Print books in the past for relatives, and hearing how much easier it is for them to read — for a variety of reasons — I wanted to provide the option for them to access what I wrote.
Depending on your distribution resources, having a Large Print version available may appeal to libraries. This I have not yet tested… time will tell.