Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Olympus E-420 and Nikon S640 side-by-side from the front, back and top in their relative dimensions. Olympus E-420 has external dimensions of 130 x 91 x 53 mm (5.12 x 3.58 x 2.09″) and weighs 426 g (0.94 lb / 15.03 oz) (including batteries). Nikon S640 has external dimensions of 91 x 55 x 21 mm (3.58 x 2.17 x 0.83″) and weighs 130 g (0.29 lb / 4.59 oz) (including batteries).
Below you can see the front-view size comparison of the Olympus E-420 and the Nikon S640. Nikon S640 is clearly the smaller of the two cameras. Its body is 39mm narrower, 36mm shorter and 32mm thinner than Olympus E-420. As you can tell, actually this is not a fair size comparison since Nikon S640 has a built-in lens but Olympus E-420 doesn't. Once you put a lens on E-420, its thickness will increase significantly.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Nikon S640 is significantly lighter (296g ) than the Olympus E-420 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on E-420, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Olympus E-420 nor Nikon Coolpix S640 provides any type of weather sealing in their bodies, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Olympus E-420 and Nikon S640's LCD screens have the same diagonal size of 2.70 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.