I agree, anything smaller then 12" is unsuited...

I am running band in a box, ableton live, cubase, NI komplete and many other vst's and some music reader apps on my HP Z1 (G2) thats not a tablet but an all in one ( like the imacs) but it has a 27" full hd touchscreen..... Size does matter, but its not heavy and since its on a stand, weight is not an issue either...

But to keep things readable and touchable, i would say atleast 12"..