Largest I did was 180 x 120 cm and 120 x 80 cm and it looks great.
I made it for a therapy center.
Note that the object fits in a palm...
I made the picture in the sand of a riverside on a cloudy day with the prime 01.
Even the larger one looks good.
I guess there won't be any viewer running around with a magnifying glass counting pixels.
But they could count grains of sand :-)
Again, may be there are cameras with larger sensors and bigger resolution..
I do not see a reason to switch the camera and to be honest, right now there would be only one camera to take a deeper look into and that is the Fuji X-E1 with the prime 35 mm lense.
But as I am on a tight budget the Pentax Q makes me happy :-)
regards
Oliver