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 Canon 400D and Sony S2100 side-by-side from the front, back and top in their relative dimensions. Canon 400D has external dimensions of 127 x 94 x 65 mm (5 x 3.7 x 2.56″) and weighs 556 g (1.23 lb / 19.61 oz) (including batteries). Sony S2100 has external dimensions of 98 x 61 x 27 mm (3.86 x 2.4 x 1.06″) and weighs 167 g (0.37 lb / 5.89 oz) (including batteries).
Below you can see the front view size comparison of Canon 400D and Sony S2100. Sony S2100 is clearly the smaller of the two cameras. Its body is 29mm narrower, 33mm shorter and 38mm thinner than Canon 400D. As you can tell, actually this is not a fair size comparison since Sony S2100 has a built-in lens but Canon 400D doesn't. Once you put a lens on 400D, 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.
Sony S2100 is significantly lighter (389g ) than the Canon 400D 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 400D, the total kit will weigh much higher.
Unfortunately neither Canon EOS 400D (EOS Digital Rebel XTi / EOS Kiss Digital X) nor Sony Cyber-shot DSC-S2100 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 DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Sony S2100's 3.00" LCD screen is slightly larger than Canon 400D's 2.5" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.