bigger tends to be heavier, something to consider...

I have a 20-60x66mm, but rarely can I use 60x, my opinion with most scopes you either need huge glass up front to use 60x or very clear skies. I believe my glass is 66mm.

Also the longer the range, the more junk there is in the air that you have to view through. I typically find about 45x being the most/best mag I can use.

So as you stated 6-8miles to judge horn size, 45x would be enough but 50mm glass I don't think will be. I'd say in that case get at least 66mm glass if not 80mm. Also the larger the glass the higher the mag you could use.

So you may want to consider something in the 20-60x80mm range.
I still have an old junker spotter, cranked at 45x my 14x leupold could see clearer than that baby could at just about any mag, let alone 45x. So I think quality is the name of the game. Also at higher mags the image can go dark if not using quality or large enough optics.

I think most spotters tend to either come with 66mm or 80mm glass, those are the typical 2 options. For a backpack 45x with 66mm glass would be plenty good.