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 A1100 IS and Canon 20Da side-by-side from the front, back and top in their relative dimensions. Canon A1100 IS has external dimensions of 95 x 62 x 31 mm (3.74 x 2.44 x 1.22″) and weighs 150 g (0.33 lb / 5.29 oz) (including batteries). Canon 20Da 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 A1100 IS and the Canon 20Da. Canon A1100 IS is clearly the smaller of the two cameras. Its body is 49mm narrower, 44mm shorter and 41mm thinner than Canon 20Da. As you can tell, actually this is not a fair size comparison since Canon A1100 IS has a built-in lens but Canon 20Da doesn't. Once you put a lens on 20Da, 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 A1100 IS is significantly lighter (620g ) than the Canon 20Da 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 20Da, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Canon PowerShot A1100 IS nor Canon EOS 20Da 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 A1100 IS's 2.50" LCD screen is slightly larger than Canon 20Da's 1.8 screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.