ski length will depend on your height and weight, type of ski, ability, and the terrain you plan to be on.
i ski bd crossbows, which are fairly fat and very stiff. i'm 6'1", 175 pounds and ski pretty agressively. i use 179s, and i have never wanted anything longer in the backcountry and occasionally shorter when i've been in really tight trees and chutes.
since you say you are intermediate, i would go for the shorter of two skis if you have a question about which to go with.
post up the stats i mentioned and the ski you are thinking about and i'll try to help.
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