First of all, we will have to relate image size (pixels) and resolution (pixels per inch, ppi).
A digital photo is made up of millions of tiny dots called pixels. The more pixels in a picture, the bigger the image becomes.
Resolution of an image is measured by pixels per inch or ppi. The higher the ppi value, the better quality print you will get. 300ppi is generally considered the point of diminishing returns when it comes to ink jet printing of digital photos.
When determining how many pixels you need, it all depends on how you will be using the photo and what size it will be printed. If you are exporting photos for web or screen display, you will need around 75ppi (pixels per inch), for home printers 140 – 200 ppi, and the best quality prints will require 300 ppi.
As a rule of thumb, the number of pixels required for exporting a photo is determined by multiplying the horizontal and vertical print sizes in inches by the pixels per inch. The table below is a starting point:
Now, also mind the aspect ratio. Make sure your photos are in the cropped correctly for the print size you want. You can use the crop tool in PhotoDirector. For example, the photo below was taken in an aspect ration of 3:2 from the camera, if I want to output the photo in a 5"x7" print, I will have to adjust the dimensions by using the crop tool. Since I wanted the lighthouse to be the main subject I adjusted the frame to the far right.
To export the photo for a high quality 300 ppi print, I start with 2100 x 1500 pixels.
When sharing photos online, 800 by 600 or 1024 x 768 pixels is a good average size to go with. You can export to higher quality if your purpose is creating photo slideshows to view from high-resolution monitors.