What is the best way to determine what cricket bat size to use? This is a general question but my specific case is for my 8 year old son.
Good question. A common mistake is to select a bat that is too big for a young player. The temptation may be to buy a large bat for the player to "grow into", to save money as much as anything else, but it's definitely not a good idea. The bat should feel comfortable to and manoeuvrable by the player.
Bob Woolmer's Art and Science of Cricket has this to say on choosing a bat for a young player (p.101):
The ideal length for a bat is the inside measurement of the batsman's trousers [...]. Bats that come up to a boy's hip or waist will do irreparable damage to his technique - not to mention his pleasure and interest in the game.
The weight of the bat must also be carefully considered [...] children over the age of seven fare best with bats that weigh no more than a kilogram. A good yardstick is to ask the child to lift the bat in his non-dominant hand (i.e., his left hand if he is right-handed), and to hold it out horizontally with his shoulder for one minute. If he can do this without strain or difficulty, the weight is correct.
Personally, I would err on the side of caution and opt for a bat that is lighter rather than heavier. The last thing one wants is a bat that impedes the development of good technique.