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 D3100 and Sony W560 side-by-side from the front, back and top in their relative dimensions. Nikon D3100 has external dimensions of 124 x 96 x 75 mm (4.88 x 3.78 x 2.95″) and weighs 505 g (1.11 lb / 17.81 oz) (including batteries). Sony W560 has external dimensions of 94 x 56 x 19 mm (3.7 x 2.2 x 0.75″) and weighs 110 g (0.24 lb / 3.88 oz) (including batteries).
Below you can see the front-view size comparison of the Nikon D3100 and the Sony W560. Sony W560 is clearly the smaller of the two cameras. Its body is 30mm narrower, 40mm shorter and 56mm thinner than Nikon D3100. As you can tell, actually this is not a fair size comparison since Sony W560 has a built-in lens but Nikon D3100 doesn't. Once you put a lens on D3100, its thickness will increase significantly.
Here is the back view size comparison of the Nikon D3100 and Sony W560.
Now lets look at the top view comparison of Nikon D3100 and Sony W560.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Sony W560 is significantly lighter (395g ) than the Nikon D3100 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 D3100, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Nikon D3100 nor Sony Cyber-shot DSC-W560 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
Nikon D3100 and Sony W560's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.