Perfect size is purely subjective. It may be perfect for you but I want a bigger one. I'm not sure how them making a bigger phone would negatively impact your experience with the X. All the other stuff you mention wouldn't need to change.
14 hours is good but would you complain about more?
I would complain, if it meant only one phone option for the category of premium phone from Motorola.
It's size, weight, and overall feel - not to forget about performance - is basically perfect.
What if the plans are for only maintaining a global two-phone lineup? A low-end and a premium phone. I wouldn't want the premium phone to be a monstrous brick.
And I, more than anything, want Motorola to focus on having a quality portfolio of hardware that affords them to give Apple-level attention to detail on each product. I don't want them to cheapen their lineup by offering a billion variations of their main phone - and I think the streamlined, quality lineup is what Google and the new management at Motorola is really pushing for. It'll allow for a more profitable company with a better relationship with their customers, as opposed to one focused on carriers like previously.
That said, perhaps they plan to, or at least have the capability, to offer a Moto XL (that's not following HTC's naming convention, at all!), or perhaps a Moto X6 for 6" face, or a Moto X+. Perhaps better would be to have a completely separate brand for the tabphone size. If they plan to have a certain naming convention for phones, and a naming convention dedicated to tablets, that would be something.
I don't want to see a Samsung approach, however. I'm not an investor, but I want good brands and tech and companies to succeed, and I don't want Motorola to risk going overboard. A carefully curated product lineup has the most potential for success. It helps on the accessories front to have fewer different devices and makes it easier to plan product refreshes: X-2, G-2, or whatever they plan to do for new models. I sincerely hope they are making brands to last, simple ones at that. Razr M, Razr HD, Razr Maax and etc, I found those stale - and too reminiscent of Motorola's dependence on Verizon. More importantly, it keeps the brand on the mind of the consumer, which is incredibly important.
And if they can make a decent market penetration, word of mouth and joy of use of the current products, and first-hand experience of internal hardware quality (reception quality and voice activation/active display), this first round could fuel some incredible success if Motorola gets products to worldwide destinations much faster next time around and really markets the hell out of the phones.
Ease of finding a wide-array of unique accessories that work with your phone, however, really demand careful attention to the accessory industry, which in turn means being careful with phone designs to ensure compatibility between refreshes (if desired, it helps but is not required), or, better yet, simply have enough market penetration that accessory makers see the obvious potential.
Samsung has a billion devices, sure... but quality is not on the Motorola level. Whether Samsung could achieve that by being a little more reserved in the product lineup expansion, is anyone's guess. I think Samsung is just Samsung, in that hardware and build quality are not currently on their list of true concerns for much of the markets they are in. They make some decent and reliable equipment at the high-end of each category, but they're biggest focus, more than anything else, has simply been to reach more and more markets, with more and more products. Every now and then they produce a gem compared to other available offerings, but that isn't always saying much either. They aren't low-end - I could never argue that, unless it's specifically a budget, low-end device. They make "solid" products - just, in almost every category, there are usually better manufacturers (not comparing a Samsung device to an Oppo device - rather, Sony, Panasonic, Philips, LG, etc) that have a far more "premium" product.
I digress, but I figured someone would make a point of Samsung's mobile product lineup. I'm having none of that.
In the end, Motorola will probably release a tabphone-sized product. They'll either call it a small tablet or a phone, and give it cellular voice capability. It is an important market category that they shouldn't miss out on.
For me, it was incredibly refreshing to see a premium phone release at a size smaller than basically all other premium phones - which is one of many reasons that led me to buy it. But I do recognize it's a market that is seeing a ton of interest.