Anybody know generally what size X 15 holster would fit a 5 inch n-frame Smith and Wesson revolver?

Bianchi only offers the X15 "N" frame holster in 6 1/2 or 8 3/8 barrels.
To fit you'd need the 6 1/2 inch version............

SKU 1015614   PART NUMBER 12365  MODEL X15.

Truth be told, the X15 like many vertical shoulder holsters is more or less "One size fits most".

Other makers may offer a closer fit for the shorter barrel.

Just tried my 625-8 in my large X15 holster.  Looks like about 2.25" is surplus.  Probably needs that extra 0.75" to fit well with a 6 1/2" barrel.  It is not terrible, and no one would notice.  I can see where it would bother some people, though.  It is difficult to conceal a 5" N-frame for most people, and even though I don't much care for shoulder holsters, it is a viable concept.