Wood Screws

[nogera@juno.com]

This question regards wood work but my wood tools are in my shop and you
guys are the most knowledgeable around so :

I remember a rough rule regarding the length of a wood screw, the
penetration into the base should be  1 to 1 1/2 times the thickness of
the piece being held  to the base by the screw .  Is there a rule as to
the size screw ?   When should you use a # 10  screw rather than a #  8 ?
 

Thanks  

Bob Nogueira