The end quality depends on the way in which any reduction in size is handled. Copies of the originals should always be archived to ensure you have a copy of the high-resolution original before it is edited. Most printers have a print resolution much higher than the screen resolution on a computer, so pictures should be printed at the highest resolution possible. If the picture is downsampled for the screen and then printed, the print quality will suffer. If by size reduction, you mean the size of the data file, there is no way to reduce the file size any significant amount without a loss in quality. JPEG is a lossy compression format: the more compression, the smaller the file, the lower the quality. There are lossless compression formats, but any reduction in file size is limited. If you have shot the photographs in TIFF mode on the camera, converting them to a lossless compression format will reduce the file size. The same is also true for RAW photos, but you should archive the RAW image before converting it to a lossless format. If you want to reduce the size of a picture to fit a computer screen (e.g., for a website), the first step would be to crop the original exactly as you would like to see the final image. It is better to crop first and then downsample, rather than vice-versa. Once the image has been cropped, you can use the resampling function in your photo editing software to resample (downsample) the image to a lower resolution. This process will always degrade the image quality. In general, the better the software quality, the better the end result.

You can most probably change the image size on your camera, if you are sure you will not need them for large hi-res prints. Have a look through your menu or manual.

Easy, compress them with Image Compressor http://www.image-compressor.com Very good picture quality and compression ratio for photos taken by 2MP or more digicam.

8-2-2017 Any image viewer can change the size of an image. It is arbitrary.