The largest print available is 10ft x 10ft print, however, you can combine several 10ft x 10ft prints to make a larger print. How big you go depends on how much space you have and your preference for whitespace/picture balance.
Most custom print jobs are considered to be "large format" if they have at least one side over 36 inches (3 feet). Artisan HD can print up to a 10 x 10 feet and for larger prints can create several 10 x 10 prints that can be put together (most popular for wall coverings or extra-large signage of some type).
Whether or not your image can translate into a high-definition print depends on the pixels per square inch (PPI) of the image file and the dots per square inch of the printed image. At least 300 PPI is best, though depending on the size and image that can range between 150 - 300 PPI. DPI must be at least 300 DPI with the best definition achieved with printers that can produce between 300 - 600 DPI.
Vinyl is one of the most popular materials used in custom print jobs. This is because it's versatile (indoor or outdoor), durable (can last many many years), and allows for high-definition printing. For signage, this allows you to get signage that checks all the boxes.