Kind of a vague question really. Some ride straight on grass with no snow. Just need to concern with cooling and lubrication as erick wrote. If you don't know the area then even 24" of fresh powder can be dangerous. Think about the elusive stump that is 20" high. Sure enough when you go over that in light fluffy stuff, you'll feel it. Skid plates help for that sort of thing. Flat trail riding, known area, no foreign objects, 6" would rock.
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I think.... therefore I sled
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Two '94 Indy XLT SKS