Working through recommendations for a family member pushed me to look into this a lot. Font size alone isn't the full story, but a bigger point size is obviously a starting place. Most standard print novels are around 10 or 11 point, which is a real struggle. For a physical book, moving to a 'Large Print' edition usually means 16 to 18 point font. It's a game-changer, but the availability is spotty unless you're looking at classics or bestsellers.
Where digital reading wins is the customization. On an e-reader or tablet, you can bump the font size up as much as you need, and the text reflows so you're not constantly scrolling sideways. A lot of visually impaired folks I've talked to set theirs to something like 24 or even 30 point. The real accessibility hero, though, is adjusting the font itself—something like a sans-serif typeface like Arial or OpenDyslexic, with high contrast and extra line spacing, often helps more than just raw size increase.