Spoonflower offers custom digitally printed fabric for only 22$. Choose “performance knit” as your fabric, and the yard size is a generous 54″. You can print any size poster by centering the image and trimming later on. Check out how nice the text looks in this shot – take into consideration my potato camera!
Your poster should be pdf, png, or tif, and at least 300 dpi. I like to convert my pdfs into 600 dpi and compress so they are under the maximum 20 mb.