Ski length depends on a lot of things which in turn depend on the skier and where / how you ski.
If you really like the 171 Filthy Rich and don't feel like you need more stability, then a pow ski in the 172-180 range would probably be good. In powder a bit of extra length (along with the extra width of a wider ski) helps with floatation.
The tradeoff with longer skis is that they can be more difficult to turn and maneuver. But strong skiers often prefer the longer length cause you usually get a bump up in high-speed stability. While it's a good call to go with a ski that's shorter than you are when you first start skiing (cause it's easier to handle), many skiers end up skiing skis that are taller than they are as they progress since longer skis often feel more stable at speed (e.g., I'm 5'8" 150 lbs and prefer skis in the 184-190 cm range). Though this isn't as important if you stick to more moderate speeds, the park, and / or tighter terrain — then shorter lengths make more sense since you won't really be taking advantage of longer skis' higher top ends.
Another thing to take into account is rocker — rocker decreases the effective edge of a ski and makes it feel shorter than it actually is. Most modern pow skis feature more rocker than the average park ski, so rockered skis can sometimes feel a bit shorter than their length suggests. Skis with tapered tips and tails can also decrease the swingweight and effective edge, again making them feel shorter than a ski with a less tapered shape.
If you love your 171 Filthy Riches, then I'd recommend the 176 Kartel 116 for a dedicated pow ski, or the 176 Kartel 108 for a ski that you can use on both deep days and days when there's just a bit of soft snow. Both are great skis in their own right, and if you got along well with the Filthy Rich, I bet you'd like the Kartels.
Other skis that might be worth checking out:
175 or 185 cm Armada ARV 116
174 or 184 cm Moment Wildcat
175 or 181 cm Faction Prodigy 4.0
176 or 182 cm Black Crows Anima
