Pad sizes, shapes and dimensions depend not only on the component you are using but also the manufacturing process used to assemble the board. There is an important parameter known as the pad/hole ratio. It is the ratio of the pad size to the component lead hole size in that pad. As a simple rule of thumb, the pad should be at least 1.8 times the diameter of the hole or at least 0.5mm larger. This is to allow for alignment tolerances on the drill and the artwork on the top and bottom layers.
There are some common practices used when it comes to generic component pads. Pads for leaded components like resistors, capacitors and diodes should be round, with around 70 thou diameter being common. Dual in line (DIL) components like integrated circuits are better suited with oval shaped pads (60 thou high by 90-100 thou wide is common). Most surface mount components use rectangular pads with circular ends and the pads should not be any wider than the component itself. As a general rule, use circular or oval pads unless you need to use rectangular.