I can't tell you the exact size in inches but 6 million pixels should give you excellent photos up to A3 size. Providing you set your camera to 3008x200 and "Fine". At this setting you will get less photos on your memory card but they will be best quality.
You will probably see no or very little difference by going to 10 million pixels. What is important is not the number of pixels but the size of the sensor that records the photos.
See above for an explanation of pixels