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 SD940 IS and Olympus E-P3 side-by-side from the front, back and top in their relative dimensions. Canon SD940 IS has external dimensions of 89 x 55 x 20 mm (3.5 x 2.17 x 0.79″) and weighs 120 g (0.26 lb / 4.23 oz) (including batteries). Olympus E-P3 has external dimensions of 122 x 69 x 34 mm (4.8 x 2.72 x 1.34″) and weighs 369 g (0.81 lb / 13.02 oz) (including batteries).
Below you can see the front-view size comparison of the Canon SD940 IS and the Olympus E-P3. Canon SD940 IS is clearly the smaller of the two cameras. Its body is 33mm narrower, 14mm shorter and 14mm thinner than Olympus E-P3. As you can tell, actually this is not a fair size comparison since Canon SD940 IS has a built-in lens but Olympus E-P3 doesn't. Once you put a lens on E-P3, its thickness will increase significantly.
Here is the back view size comparison of the Canon SD940 IS and Olympus E-P3.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Canon SD940 IS is significantly lighter (249g ) than the Olympus E-P3 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 E-P3, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Canon PowerShot SD940 IS / Digital IXUS 120 IS nor Olympus PEN E-P3 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 Mirrorless Cameras with Weather Sealing
LCD Screen Size and Features
Olympus E-P3's 3.00" LCD screen is slightly larger than Canon SD940 IS's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.