UPDATE: Jon Ostrower reports that the exact diameter will be 69.4" and that the engine will also get smaller core size. I have not seen the article myself as I do not have a WSJ subscription, but Scott Hamilton has an update on the story, too.
Scott writes that a smaller core fits better under the wing. Well, yes and no - the core, meaning the HPC and HPT are physically smaller, but as the worksplit between the high and low spool changes, the LPT has to do more work now - and at the same time needs to run slower because of the larger fan. So the diameter of the LPT has to move out as described earlier. And there could really be a need for an additional LPT stage with the higher loading of the LPT. Only if the efficiency of the LPT can be kept constant the smaller core size and higher bypass ratio will do any good for SFC and fuelburn. Temperatures in the core will rise anyway - what this means for maintenance costs customers will see only a few years after putting the engine in service.
It is becoming a real saga: the fan diameter of the B737MAX LEAP-1B engine. Now we are at 70", compared to the 61" of the CFM56-7BE and the last announced 68.4". Read the story at Leeham News here:
Sorry - but, but for me Boeing is loosing credit with every change that is announced (or, as in this case, reported by a credible source). For months Boeing now said "Bigger is not better", defending the B737MAX and the LEAP-1B against the A320neo and the LEAP-1A/PW1100G with their considerably larger fan sizes.
So, is the larger 70" fan just for optical reasons? Just a joke, of course...
A larger fan needs to run slower to keep the efficiency high - or in other words: the tip speed needs to be the same as it was before. But this means that the LPT also needs a larger diameter to keep it's efficiency - or the LPT needs an additional stage. If this is beneficial in the end? It looks like Boeing looks only at fuel burn now, not at overall operating costs in the first place. I eagerly wait for the next press release from Boeing regarding MAX.