Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Nikon L31 and Sony A390 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). Sony A390 has external dimensions of 128 x 97 x 86 mm (5.04 x 3.82 x 3.39″) and weighs 549 g (1.21 lb / 19.37 oz) (including batteries).
Below you can see the front view size comparison of Nikon L31 and Sony A390. Nikon L31 is clearly the smaller of the two cameras. Its body is 32mm narrower, 38mm shorter and 57mm thinner than Sony A390. As you can tell, actually this is not a fair size comparison since Nikon L31 has a built-in lens but Sony A390 doesn't. Once you put a lens on A390, its thickness will increase significantly.
Here is the back view size comparison of Nikon L31 and Sony A390.
Now lets look at the top view comparison of Nikon L31 and Sony A390.
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 (389g ) than the Sony A390 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 A390, the total kit will weigh much higher.
Unfortunately neither Nikon Coolpix L31 nor Sony Alpha DSLR-A390 provides any type of weather sealing in their body, 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 Sony A390's LCD screens has the same diagonal size of 2.70".
On the other hand, Sony A390 features a Tilting screen
where you can change the angle of the screen to make it easier to shoot from waist or over the head levels whereas the Nikon L31 has a fixed type screen which provides less flexibility in shooting positions compared to A390.