I use a surface pro 1 with the 10" screen.
It is small but I am indulgent with myself and accept it because I love compact setup
My dev computer is a ACER R7 with an amazing 15.6" touch screen. It is very good.
27" is huge, for home players only. You don't have any excuses if you fail the button click
vArranger is auto-re-sizing and adapting to any screen. So everyone will decide.
We don't need multi touch in vArranger, but it is working. Windows 8.1 is perfect for vA.
Note : the vArranger synth , and the vst host are included with vArranger