It will be interesting to see the final optical design.

If the prime on launch corresponds to the existing silhouette in the roadmap, then back-of-the-envelope ratios (calculated off the 100-400 S) puts this 800 f6.3 at approx 140 x 315 mm overall dimensions.

Without a reliable Phase-Fresnel scaling factor for telephoto mass, the weight is harder to guesstimate, but 58% of 4.59kg = 2.66 for a hypothetical 800 f5.6, so a 800 f6.3S PF can be expected to weigh < 2.5kg, perhaps even 2.3kg. Consider the 300 PF @ 89 x 148 mm, 0.755kg [58% of the 1.295kg 300 f4AFD] and 500 PF @ 106-237 mm, 1.46kg.

Compared to the 4.59kg 800 f5.6E FL of 160 x 461 mm, this 140 x 315 mm 800 f6.3S PF has indeed undergone a major slimming! One often reads forum posts stating a 3kg telephoto is about their maximum comfortable handholding weight, but I manage short durations with 5.3kg of 400 f2.8E & D5 with TC2, so I another who finds that handholding the 500 PF is a doddle.

I also learnt some years ago how a heavy DSLR is a key to balance a long heavy lens. Poor weight distribution can be a killer IME if a telephoto is 'front-end heavy', this makes a 300 f2.8G VRII harder to handhold than the much better balanced 400 f2.8E FL. Besides this 800 f6.3 PF trimming major weight for handholding, hiking and flying, we also know the Z MILC AF works reliably at lens speeds slower than f8. So the superior image quality of the TZTC14 and ZTC2 underscore even more bonuses.