SIZE
Any 600/5.6 lens will have an front element (entrance pupil) of 107mm, the same size as a 300/2.8 lens, so the overall diameter will be about the same. The length of the 600PF is given as 330mm, compared to 268mm for the AF-S 300/2.8 models. So while the lens will be substantially smaller and lighter than the 600/4, it will still be larger (longer) than the 300/2.8, not an insubstantial lens.
WEIGHT
Telephoto lenses with similar size diameter include:
AF-S 200/2 VR - 2930g
AF-S 300/2.8 VR - 2900g
AI-S 400/3.5 - 2800g
AI-S 600/5.6 N - 2800g
Most other similar size AI, AF and AF-I telephotos have similar weights, at slightly less than 3kg (see
http://photosynthesis.co.nz/nikon/specs.html#300). The shorter teles have thick/heavy elements in a shorter barrel, the longer lenses have thinner/lighter elements in a longer barrel, so the weight is pretty even across this group in spite of the difference in focal length.
Fresnel lenses are much thinner than regular lenses which could permit some weight reduction. The Fresnel lens plays a similar role as Fluorite lenses in controlling CA and reducing weight, so I don't think this lens will also have Fluorite lenses which could reduce weight further. Using lighter materials in its construction could also help. Overall I would expect a weight somewhere between 2-2.5kg - due to the size of the lens I don't expect such a dramatic reduction in weight as we saw with the 300/4PF.
Probably the best comparison is the Canon 400/4 DO lens, a similarly sized lens also with a Fresnel element. It is 233mm long (100mm shorter), weighs 2.1kg, and cost is 6900 USD. I wouldn't be surprised if the Nikon is similar is weight and cost
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