Size of file change when i resize the photo, does it effect photo quality ?
YES. That is why you need to save it as a new name, so you don't overwrite your original file.
Save your smaller images as JPEG's, since they are the standard of the industry for posting and sending via email.
All JPEG images lose quality every time you do something to them, even rotating them 90 degrees.
If you want to reduce the loss of quality, save them first to either PSD or PNG files and after you have done all the manipulation, save a final image in JPEG.
Size of file change when i resize the photo, does it effect photo quality ?
yes it will, anyway, every time you re-save a jpg file it loses information, re-sized or not, doesn't matter.
There are many ways to re-size a shot, most photo editors use a bicubic resizing. This is subject to huge loss of quality. Best way to resize in any direction is a fractal calculator like fractal design print pro or alien skin Blow Up. Such software recalculate space and relocate content of every pixel without major loss in quality. For sure, you cant' squeeze the content of a 12 mpix shot in a 3 mpix reduced size without any loss. Just in fractal calculation it makes the best out of it and loss is not visible. Also, run the save as command after and keep your original picture the way it was.
No.
