Hi. A lot depends on what method of application you are going to use. If you are looking at highly detailed print, then you are looking at using a print resolution of 300 dpi. The canvas size is dictated by the capabilities of your print method. For example if you only have an A4 size printer (8"x12"), there would be little point in setting up Photoshop for an A3 print (12"x16"). Know the world has gone mad with ultra high resolutions, but for fabric 300 dpi will create a good, vivid, saleable design. But If you would use vector rather than raster graphics, it would be even more better.
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