While I prefer the laptop and smartphone combo since I like to be mobile, I've come to realize that when at my home or work office I do prefer a dual monitor setup. Mainly because my workflow at the desk benefits from the dual monitor setup, while if I am on the road my laptop is what I need.
The work I do at my home, or work, offices benefits of dual monitor setups since I need to compare information or check the info on different apps often. Meanwhile on the road, I'm just doing stuff than a single monitor is enough. Besides, my ideal dual monitor setup is my laptop monitor along side a 27" monitor as my primary one while I'm my offices desk.
For my ideal laptop monitor size, it would be the 15.6" one. It gives me the balance between screen size and portability I seek on a laptop. I tried a 13" laptop once, and I found it to be to small for my use, and larger screens on laptops make them to bulky for to carry around on daily basis.
On my desk, a 24" or 27" monitor gives the display real state I prefer on my primary display, while the 15.6" inch of my laptop is the perfect second display for my perfect dual monitor setup. I prefer having a smaller display for referencing, while I work on the larger display. Though, I could happy work with both displays being 15.6".
It has been about 4 years since I starting working with a dual monitor setup at my desks, and I've found that this setup is the best for my workflow. I prefer to have one app per display, so a wide aspect display doesn't seem to be a good option for me, but I still got to try it out.