The 919 uses 296mm rotors, 929 uses 310mm rotors, and the 954 uses 330mm. It makes no difference to the calipers however, because Nissin uses the same general pattern for the mounts and E center, so the only differences are the piston diameters and the location of the caliper mounts on the forks. I know this because I fit up a pair of CBR600 calipers (310mm rotors) I got off Ebay for a song while swapping out pads and replacing the front tire, and they dropped right into place. Didn't leave them on -- the 919's brakes are quite adequate for the street for a braking minimalist like me and the calipers were earmarked for a door (as it were) prize on one of my periodic rides.
Thanks for chiming in, your input is highly valued.
I bit more of your time please.
My intent is to stay with either my already reworked 919 front end, or possibly go to a reworked 4Fi front end (purely to get the low speed compression adjuster that they have).
I'm toying with 929 calipers and rotors, or 954 calipers and rotors, or maybe even 954 calipers on 929 rotors - final decision being partially E Bay dependent.
F4i calipers are a simple bolt on, regardless of whether it's a 919 or F4i fork legs, I'm straight on this point.
But I get the feeling from your comments above, that the caliper mounting lugs and tapped holes on 919/F4i legs, while in the same pattern for a 929 or 954 caliper, are likely not correctly positioned on the fork leg to be able use 929 or 954 rotors.