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 Nikon L31 and Pentax K-x side-by-side from the front, back and top in their relative dimensions. Nikon L31 has external dimensions of 96 x 59 x 29 mm (3.78 x 2.32 x 1.14″) and weighs 160 g (0.35 lb / 5.64 oz) (including batteries). Pentax K-x has external dimensions of 123 x 92 x 68 mm (4.84 x 3.62 x 2.68″) and weighs 580 g (1.28 lb / 20.46 oz) (including batteries).
Below you can see the front-view size comparison of the Nikon L31 and the Pentax K-x. Nikon L31 is clearly the smaller of the two cameras. Its body is 27mm narrower, 33mm shorter and 39mm thinner than Pentax K-x. As you can tell, actually this is not a fair size comparison since Nikon L31 has a built-in lens but Pentax K-x doesn't. Once you put a lens on K-x, its thickness will increase significantly.
Here is the back view size comparison of the Nikon L31 and Pentax K-x.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Nikon L31 is significantly lighter (420g ) than the Pentax K-x 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 K-x, the total kit will weigh much higher.
Unfortunately neither the Nikon Coolpix L31 nor Pentax K-x 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 Compact Cameras with Weather Sealing Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Nikon L31 and Pentax K-x'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.