You need to plan and prepare for a few different variables, one of the biggest being the color of the garment. If you are already a sublimator, you know that the process likes white the best. The inks are transparent, so transferring to anything but white will change the color of the images. This is something you need to prepare for — do some testing before production, especially if your customer is color conscience.
The next variable to consider and plan for is the size of the garments you will create. An image that works on a large shirt might not look right on an extra small or conversely a double extra-large. Size your image appropriately or print several different sizes for the garments you create.
To wrap up with artwork for apparel sublimation, you need to have the right size artwork files. I suggest making sure you have a minimum of 150 dots per inch (DPI), with 200 DPI being a better file size to shoot for. Remember this DPI is for the final size of the shirt being printed, so make sure you have artwork prepared for all the different size shirts you might create.
—Aaron Montgomery, MontCo Consulting, 2 Regular Guys