An 85mm brake is designed specifically for frontside and all-mountain skis with a waist width between 73mm and 83mm. If your skis fall in that range, this is likely your best bet—it gives you enough clearance for the brake arms to deploy properly without hanging so far below the ski that they drag in the snow during turns.
The key is matching brake width to your ski's waist (the narrowest point underfoot) plus a little extra room. Going too narrow means the brake pads sit on top of the ski instead of dropping below it, which defeats the whole purpose. Going too wide creates annoying drag and can catch in soft snow.
If your ski waist is right around 80-82mm, the 85mm brake is a perfect fit. For skis at the lower end of the range, like 73-75mm, you could also consider a 95mm brake, but expect slightly more drag in deep carves.