First Enter your Gauge in Garment Designer after you have drafted the shape of cap you want, and generate Shaping instructions. Write down your total number of Stitches wide by your total number of rows Tall.
Open a new document in Stitch Painter and set up the total number of rows Tall by the total number of Stitches wide you want the document to be. If you want to design the full cap you would multiply your single width by your total number of pieces. I chose to just do half so I multiplied it by 3. I ended up with 54 x 57 grid size.