Soma is arguably the best landscape artist around here. And I learned just the other day that he’s doing this professionally. 
That picture isn’t the best example of his work, but it’s the one where he commented on resolution.
What I found quite interesting about it is that he keeps the resolution reasonable because of performance issues. So if you want to use higher resolution you might have to restrict yourself to fewer layers.
My tablet screen is 222 dpi (13") and my main monitor is 93 dpi (32"). 72 dpi was probably a good recommendation a decade or two ago… maybe? I’ve heard that gamers use screens with lower resolution so that they can get higher frame rates, so perhaps 72 dpi is still relevant.
Another thing to consider is what kind of painting you do. If it’s simulated watercolors, the edges may be pretty soft anyway and scaling the picture up won’t be all that noticeable.