The size of the board refers to the width. Depending on how old your sons are you will choose a differnt size. Then after that the size is a matter of preference. I have found that 7. 5 is a good all around size.
The standard size for a skateboard is around 18-24 inches. It depends on what type and what the skateboard will be used for. Normally all skateboard are around the length of 2 feet.