question on picture size?

I always shoot with Medium Size as I do not need larger size images. I was reading Scott Kelby's book yesterday and I understood that larger size images will have better quality, Is that true?

I thought pixels are not directly proportional to the IQ. My understanding was M size is 5 mpixes and L size is 10 mpixels. correct me if I am wrong.

That is correct. The medium size is only using half of what your camera is capable of, therefore it is lower quality (in this case meaning you could print a larger print if the image is set to large or you could zoom further in to the image/crop more before you start noticing quality loss)

For what it's worth, disk space is cheap, flash memory is cheap, theres limited reasons to not shoot in the highest quality you can

I only ever shoot in RAW as this ensures the utmost in quality for photos. I can later resize if needs be.

With memory being as cheap as it is I can see no compelling reason to only use half of what your camera is capable of.

Agreed - memory is cheap ... unless you seldom move the pictures OFF of your camera to another storage device.. Then space may be an issue for you.

As everyone else said... Memory is cheap, and i'm with michaelgreen78, RAW is the best.