How much of a canvas size is too much for digital painting

I was just wondering for photoshop how much pixelsfor canvas size everybody is used to using?

I use photoshop (previously 7, CS3 in office, recently upgraded to CC) to draw comic strips, and quite often I just keep expanding the canvas vertically as I draw more panels.Now, of course common sense would dictate that the larger the file the slower your photoshop gets. I just noticed today that one of my files is getting sluggish, and yup, the height has gotten to 14000 pixels. Better crop that thing before it gets to the point I can't even open the file anymore. So I was just wondering if there was any rule around as to how big a file can get before it's too big to operate on photoshp.