It took me awhile to figure this out, but when working with a pattern, check to see what size that pattern has been made. For example, many that I have been using are done at only 128 pixels. If I put this pattern into my 12x12 layout, it just doesn't look right. So, start with a canvas size the size of your pattern. Fill and do what you want. Finally, resize to 3600 pixels for your 12x12 size!