I see that a few companies (Orange, Singular…) don’t offer a small (15″) option, but do in 650b. Where as people like Niner do.
Is it more correct to spec frame size along with wheel size for shorties (a bit like cranks?)
Niner don’t offer a small 650b option because being exclusively 29er is their USP. More and more companies do seem to be stopping production of small and xs 29ers in favour of 650b. I suppose it makes sense, taller rider, higher centre of gravity, bigger wheels, more stability and more bb drop to lower the cog. Smaller rider, not such an issue.
Personally I’ve never been a fan of 29ers. I found that the hard things which I want to master in mountain biking, cornering, pumping, drops, jumps etc are made more difficult and less fun by 29ers. And the easy things, rolling over roots and holes, are made a bit easier. They’re certainly not for everyone.