I have to use 150% scaling on a 2560x1440 resolution 27" touch screen - if I run it at 100% I find the items too small to use effectively with touch. So you may be better off sticking at 1920?
As far as I can tell Sonar X3 mostly behaves OK with Windows 10 scaling - apart from the Cakewalk TruePianos plugin - whenever I insert that plugin, it tries to force the scaling back to 100% (and then gets very confused and I have to restart Sonar).

Perhaps, rather than having a higher resolution monitor, it would be better to have more monitors. I use 3 monitors, where normally on the central monitor I leave the track view, on the lower resolution monitor I leave the console view and on the third monitor I work with Piano Roll View, among others.

I also use 3 monitors:

• 2 old 19 inches side by side : 1 for track view + 1 for PRV/notation
• 1 newer 23 inches at 1920x1080 on the desk with a smal angle : Console view. It feels as if I had that good old console in front of me. Too bad it is not touch sensitive ! 

A great thread on the move to larger 4K screens.  I will be updating my three 27/28 inch hd screens to a Phillips 40" 4k in the new year... everyone who's done this ain't going back... my tired eyes need this clarity and screen real-estate is vastly superior... 
 
May still use an hd or two for side info but for the main screen, a 40 4k seems the best bet at the moment... and can be had for under $800ish these days... Have seen some nice 50 inchers also...

In my opinion, the higher resolution screens such as UHD 4K, etc., are designed to enhance the consumer viewing experience with movies and video games.  If you go this route, the bigger the panel is, the better!
 
I ran into this with HD when I bought a 15" Dell laptop with a 1920 x 1080 HD screen.  Great for DVDs, but couldn't read anything, as text was way too small for me.
 
My latest laptop (non-DAW) is also 15", but I made sure to get a 1366 x 768 screen.  Text is normal size, but screen real estate barely holds a Sonar interface.  It is usable, but it gets very crowded.  Z3TA+2 UI does not quite fit vertically.
 
On my DAW, with 24" 1920 x 1200 and 20" 1600 x 900 screens, the Sonar GUI works well on both.
 
I think that 1920 is probably the sweet spot for 24-27" screens, but if you go higher res, make sure to scale the screen size up as well, like microapp outlined earlier.

And for anyone considering multiple screens - be aware of issues with different resolutions / DPI....
I've got a Surface Pro 3 for work (which has a stupidly high resolution - can't remember exactly what it is). I use an external 24" screen that is good (DELL) but nowhere near the same DPI. You have to find a compromise between screen scaling settings in W10. You can set different scales for each screen, but you get very strange artefacts when moving windows between screens: when a window spans both screens, the screen that has the larger proportion on it determines to scaling.  This means that as you drag from one to another there is a point when the scaling suddenly changes (which is very off-putting).
I don't know they could overcome this - I think the answer is to get multiple screens with the same DPI - but this is not easy to do.
(There is also an issue where you can set different screen scalings for docked / undocked - but they won't take effect until you log off/on - so I have to log off every time I undock which is very frustrating).

Please be aware that if you want to use multiple monitors, you may have to change the resolution as well to keep images aligned across the monitors.
 
I have a 4K monitor I use with one 27 inch touch screen on my setup and I had to fool around with Windows settings and resolutions to get them to line up and maintain the 4k screen resolution and lost the 4K advantages except that the screen graphics are clearer then when I was using 2 27 touch screens.

paulf707
You can set different scales for each screen, but you get very strange artefacts when moving windows between screens: when a window spans both screens, the screen that has the larger proportion on it determines to scaling.  This means that as you drag from one to another there is a point when the scaling suddenly changes (which is very off-putting).
I don't know they could overcome this - I think the answer is to get multiple screens with the same DPI - but this is not easy to do.

I use two screens of differing sizes (24" and 20"), but I leave everything set at 100% scaling.  When I drag from one to another everything stays the same, no artifacts or anything else odd.  I assume they must be similar DPI.  Just more dots on the larger screen; more inches = more dots.

And also, in some cases, at least in Windows 8, if you have a secondary monitor connected via HDMI on your video card, when turning off and reconnecting this HDMI monitor, all the Sonar windows will go back to the monitor 1. This is true for all other opened programs.
I have tried several solutions to this, but the only one that solved this problem was using the software DeskSoft WindowManager, which allows me to completely control the movement between monitors, also including triggers when a monitor is turned on, when in this case I send all opened windows back to their previous monitors, regaining their original positions.

I use a 4K 55" TV and its great to have a console and track view more than a meter wide in front of me :)