That's 17mm & 15mm. Nothing found regarding M9120 XTR Enduro, but it's probable that they will be the same. Some further digging reveals that the new N-series pads are cross-compatible with existing H-series finned and D-series non-finned pads currently found in Saint, Zee & XT 4-pot (and new Deore-level MT520). M9100 XC 2-pot pistons are likely to remain 22mm, consistent with other 2-pot models.Update: The original information we received about the difference in piston diameter between the Saint/Zee brake callipers and the new XT 4-pot callipers was incorrect. We can now confirm that the piston diameter is the same between all Shimano 4-pot brake callipers.
Shimano aren't listing weights for the calipers on their own, rather giving full single brake weight, comprising caliper with pads but no mount adaptor, lever, 800mm hose and 160mm recommended-model rotor with lockring & bolts. With this setup, M9100 is listed as 312g, while M9120 is 385g, vs 316g and 360g for M9000 & M9020 respectively.