At what age do Staffies stop growing?

What exactly do you mean by a "good size"? Too small? Too big? Or just average for the breed? 35cm at the withers is only 5mm below the breed standard (he’ll likely make up those few millimetres fairly quickly, as at 6 months old he’s still got some growing to do). Either way, he’ll be within the standard, possibly reaching 37cm once he’s fully grown, so yes, it’s a good size (but again, it depends on whether you consider a "good size" to be quite large or just typical).

As for the thin face, it’s hard to judge without a photo, but the person above hit the nail on the head. He won’t grow much taller now, but he’ll start to fill out and broaden. It’s like a teenager who reaches their full height by 18, yet their physique continues to develop and change well beyond that point.

Have you got a recent photo?

Normally, at around 6 months for this kind of breed, they’re nearly full-grown. He’ll still change a bit though – he’s going to fill out and gain some muscle :)