Hi Mike,
From past experience with 2.5’ drives, they tend to be a little less reliable. 3.5’ also then to offer a slightly better performance. Also consider this is going to be an always on drive so best to look at the slightly more expensive option of 3.5’ NAS based drives. Fitting wise, unless you’re moving the thing about a lot or intend to put it in a busy, vibration prone environment, I’d probably say a small amount of double sided tape would work just fine too. Neither option of drive is the most idea as mechanical drives are the most reliable but they do offer the size. Hope this helps!