What will the drawers be used for?
It has a bearing on what materials to use (pun unintentional ;D) also method of jointing the box, and proposed runner system (self made wooden slides or bought in self closers). 3/8" to 1/2" (9 to 12mm) is common practice for sides and 1/4" (6mm) and above for bases dependant on size/span and use.
Rob.