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 Leica C and Olympus E-P3 side-by-side from the front, back and top in their relative dimensions. Leica C has external dimensions of 103 x 63 x 28 mm (4.06 x 2.48 x 1.1″) and weighs 195 g (0.43 lb / 6.88 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 Leica C and the Olympus E-P3. Leica C is clearly the smaller of the two cameras. Its body is 19mm narrower, 6mm shorter and 6mm thinner than Olympus E-P3. As you can tell, actually this is not a fair size comparison since Leica C 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 Leica C and Olympus E-P3.
Now lets look at the top view comparison of Leica C and Olympus E-P3.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Leica C is significantly lighter (174g ) 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 Leica C (Typ112) 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
Leica C and Olympus E-P3's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.