The answer to this question could run for many pages especially if one begins to consider pixel size, array size (full frame, etc.) and all such factors and possibilities. In view of this, I'm going to keep it short. For those who wish to delve deeper into the subject, there's a wealth of scientific and specialist information available via the Internet.

Simply put, the more pixels you have, the larger the file size will be - larger files can produce bigger pictures. So one thing you need to bear in mind is what your intended purpose is.

Let's look at some basic options:

If you wish nothing more than to send images across the Internet (say by e-mail or posting to a picture-sharing site) then the pixel count can be relatively low, 2 megapixels or more would be sufficient. However for attachments and to allow for various 'servers', then JPEGs of, say, 1mb to 5mb would be sufficient for reasonable photo-quality.

For making prints of around A4 size then 4 megapixels can be regarded as the minimum. As a guide to .PSD file size this will be about 10mb.

For A3 prints, 6 megapixels will just about make it whilst larger pixel counts will cope easily. As a guide to .PSD file size this will be about 20mb. Anything larger than 20mb will enable images of greater size to be printed.

Remember that cropping an image will reduce the number oavailable pixels so if you have a lot of pixels to begin with, you can make more severe crops without loosing too much quality.

For more information about this topic and how it relates to file size, resolution, printing and intended purpose, please click on the link below.