Huh? Landscape is the default for nearly all monitors. It’s horizontal while Portrait is vertical.
What you can do depends on the monitors and the MBP—about yours, we know nothing. Some require eGPU to drive hi-res large external monitors while many of the newer ones can handle a large display setup without breaking a sweat. About that… eGPU is not officially supported on the Mac OS unless over Thunderbolt 3 beginning with High Sierra 10.13.4 although there are workarounds for 10.13.3 and earlier or Mojave and later.
Because of the dependence of Dorico on the keyboard, it can be a bit awkward using one that’s tethered to a side monitor. A good BT keyboard and mouse or trackpad makes that issue go away.
Otherwise, the Displays control can place any of your external monitors however you want them vis-a-vis your MBP.
I run a 43" 4k LG monitor in landscape (sideways), it’s great being able to see the notes on an entire orchestral score without having to scroll vertically.
Could you give the exact model? I currently am using 65 inch TV as a monitor. It is excellent, but I cannot use any mounter in my room, so I have a problem with the perspectives. I am considering to purchase a smaller monitor. I also think 43 inches would be appropriate.
Just agreeing with adrien and recommending the LG 43UD79-B.
True is is larger than the norm but you soon get used to it. Screen is sharp and you get used to that very quickly. The amount of screen real-estate makes typesetting a joy.
No, he means Landscape. A 40-43" monitor can show an 11x17 (tabloid) page at actual height with room for toolbars along the top (although opening the properties panel does impinge). Screen display height is 22". I bought such a monitor for exactly that reason.