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 Canon A800 and Canon 20D side-by-side from the front, back and top in their relative dimensions. Canon A800 has external dimensions of 94 x 61 x 31 mm (3.7 x 2.4 x 1.22″) and weighs 186 g (0.41 lb / 6.56 oz) (including batteries). Canon 20D has external dimensions of 144 x 106 x 72 mm (5.67 x 4.17 x 2.83″) and weighs 770 g (1.70 lb / 27.16 oz) (including batteries).
Below you can see the front-view size comparison of the Canon A800 and the Canon 20D. Canon A800 is clearly the smaller of the two cameras. Its body is 50mm narrower, 45mm shorter and 41mm thinner than Canon 20D. As you can tell, actually this is not a fair size comparison since Canon A800 has a built-in lens but Canon 20D doesn't. Once you put a lens on 20D, 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.
Canon A800 is significantly lighter (584g ) than the Canon 20D 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 20D, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Canon PowerShot A800 nor Canon EOS 20D 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
Canon A800's 2.50" LCD screen is slightly larger than Canon 20D's 1.8 screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.