That would give you an 11x7.5 inch (approx) at 200dpi.... going larger than that would really be pushing the quality of the image. If you check with most services, they will tell you what pixels you need to be for an 18x24 (probably a minimum of 3600x4800 dpi). You could try an image that large (18x24 in) but you probably will not be happy with the quality.