Yeah, a nerf to 6" would be huge. That reduces it from 16" to 12" (plus base size). That's about the scale of change I think would be appropriate.
I absolutely hate when they nerf a model to unplayable, and we have to remember she has an overall weak keyword as well.
As for Rider, how is it dropping 4 scheme markers? It can drop one itself, and if it reactivated a schemer that is at best two more? It can't ride with me and revel in the same activation, so it cant do two scheme markers per activation itself.
Plus all the Riders are strong schemers.
Personally I generally find the card draw trigger on its gun scarier to play against. Colette for example is weak in terms of card advantage, and the rider can just negate that.
That said, they could nerf Dead Rider and Mechanical Rider, and maybe Pale Rider, and I don't think that'd be unreasonable.