Michel, I'm 5'9" with a 32 in. inseam, rather long for my height, and the large fits me fine while the medium frame fits on the small size. Chris is right, our bikes run a bit on the large size by virtue of the battery length which creates a longer than standard cockpit. I would probably recommend a medium but w/o having you ride both, and get a proper fit, it's hard to say.
It's likely you could get a decent fit on a large frame though the most important dimension is your knee over pedal spindle dimension. Since you have a good fit on the Madone, take reach, saddle set-back (knee over pedal spindle) and saddle height measurements for starters, and see how they compare to the specs on the large NEO Cross (or if you have access to the bike, use that) and what adjustments would have to be made to fit (saddle fore/aft, or stem size change).
As getting a perfect is a bit less of an issue on an ebike than a standard bike like your Madone, it will still be more enjoyable if set up right. I would guess at least 9 out of 10 ebike owners never get a proper fit on their bikes (see a bike shop that specializes in bike fitting), and probably a similar percentage needs a different size stem or saddle adjustment than what is offered (similar to any road or mountain bike sold).
Hope this helps!
