The smaller the area you crop to, the lower the final resolution. After applying your crop, go to Image/Resize, select Actual/print size, and for Resolution enter 150 Pixels/inch. The Width and Height shown will then be the largest you can safely print. If it's way too small, you'll just have to crop less tightly. If it's not too far off, turn on Maintain aspect ratio, then enter the required width or height. For Resize type, choose Bicubic Resample - a form of 'interpolation', which tries to enlarge the image without making it look blocky. (The Smart size option will do the same.)
