Using pixel-pens in Fresco

A good rule of thumb is that you need your image to be 300ppi at the size you wish to print. So a file that is 3000 pixels wide will print great up to 10" wide (300ppi X 10in. = 3000pixels).

Your image translates to 27.6in x 39.4in which equates to 8280 x 11,820pixels. Unfortunately this is bigger than Fresco alllows. That said, you could work in a smaller size at the same aspect ration, but the best you'd be able to get is 167ppi at print size. Although it's not ideal, it actually might be ok. Many print on demand places use 150ppi as a minimum resolution. 

That said you could always use the vector brushes and not have to worry about resolution.

Pixel or Rastor by nature will always have that pixelated edge. If you make your canvas the size you want to print and draw your image accordingly you'll reduce issues. It will also help to increase the ppi or ppcm under pixel size in the new document tab.