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 SX20 IS and Nikon D5100 side-by-side from the front, back and top in their relative dimensions. Canon SX20 IS has external dimensions of 128 x 88 x 87 mm (5.04 x 3.46 x 3.43″) and weighs 600 g (1.32 lb / 21.16 oz) (including batteries). Nikon D5100 has external dimensions of 128 x 97 x 79 mm (5.04 x 3.82 x 3.11″) and weighs 560 g (1.23 lb / 19.75 oz) (including batteries).
Below you can see the front view size comparison of Canon SX20 IS and Nikon D5100. As you can tell, actually this is not a fair size comparison since Canon SX20 IS has a built-in lens but Nikon D5100 doesn't. Once you put a lens on D5100, 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 D5100 is 40g lighter than the Canon SX20 IS but we don't think this will make a significant difference.
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 D5100, the total kit will weigh much higher.
Unfortunately neither Canon PowerShot SX20 IS nor Nikon D5100 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 Bridge Cameras with Weather Sealing Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Nikon D5100's 3" LCD screen is slightly larger than Canon SX20 IS's 2.5" screen.