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There is so little difference between 275 and 300, literally a shot glass of volume. Neither should look too big or unnatural on your frame – go for the 300cc if that’s what your gut is telling you! I had 430cc and I have a size 6 waist (although I have size 10 hips and I’m a little bit taller than you at 5’9”) and I don’t think that mine look big at all.