Did they decide on the 66" version? The current size is 61 inches and Boeing was looking hard at 70."

Final 737 re-engine configuration down to four possible fan sizes

The trade off is that large fans make huge thrust when low and slow. At high speed big fans and nacelles are just extra drag. Think A10 and F104 and you get the picture. Here is a toy to play with:

EngineSim 1.7a beta

While reading up on this 737RE, I was a little surprised to read that Slow and Sailing were correct that there was room in the A320NEO schedule to accommodate Delta's needs. Apparently what happened is that Airbus became pretty firm in their prices after the number of orders they received. Boeing on the other hand may have been inspired to offer better discounts.

Overall the 737-900ER may have been the overlooked "ugly duckling" with Swan potential. Part of the reason for the success of the -800 was that its commonality makes it easy to lease and re-lease to another customer. The relative rarity of the -900 will decrease with Delta's order, making the airplane more accepted in the marketplace.

The 737-900ER RE might be just the ticket, with more thrust & better efficiency. If this RE is a nacelle and engine swap, seems like it could be done mid order. The other angle is, with new engines the 737-700 might become the "perfect" 130 to 150 seat jet. We did an order for 200 engines, but since the Leap X is the same manufacturer, an armchair observer would think there is a chance the order could be up-sold.