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 Casio EX-Z800 and Olympus E-P1 side-by-side from the front, back and top in their relative dimensions. Casio EX-Z800 has external dimensions of 91 x 52 x 20 mm (3.58 x 2.05 x 0.79″) and weighs 124 g (0.27 lb / 4.37 oz) (including batteries). Olympus E-P1 has external dimensions of 121 x 70 x 36 mm (4.76 x 2.76 x 1.42″) and weighs 355 g (0.78 lb / 12.52 oz) (including batteries).
Below you can see the front-view size comparison of the Casio EX-Z800 and the Olympus E-P1. Casio EX-Z800 is clearly the smaller of the two cameras. Its body is 30mm narrower, 18mm shorter and 16mm thinner than Olympus E-P1. As you can tell, actually this is not a fair size comparison since Casio EX-Z800 has a built-in lens but Olympus E-P1 doesn't. Once you put a lens on E-P1, its thickness will increase significantly.
Here is the back view size comparison of the Casio EX-Z800 and Olympus E-P1.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Casio EX-Z800 is significantly lighter (231g ) than the Olympus E-P1 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-P1, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Casio Exilim EX-Z800 nor Olympus PEN E-P1 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-P1's 3.00" LCD screen is slightly larger than Casio EX-Z800's 2.7 screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.