I assume you want to print the photo. Then, 180 ppi is the minimum resolution you need for best quality results. So, you can enlarge the photo to 300/180 x 18 = 30 inches wide. Of course, "best" quality depends on more than just the PPI, like proper sharpening and so on. For larger, you will want to uprez (increase the ppi). Photoshop has some tools for this that other like, but I find Genuine Fractals to work better.