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 550D and Panasonic ZS8 side-by-side from the front, back and top in their relative dimensions. Canon 550D has external dimensions of 129 x 98 x 62 mm (5.08 x 3.86 x 2.44″) and weighs 530 g (1.17 lb / 18.70 oz) (including batteries). Panasonic ZS8 has external dimensions of 105 x 58 x 33 mm (4.13 x 2.28 x 1.3″) and weighs 210 g (0.46 lb / 7.41 oz) (including batteries).
Below you can see the front-view size comparison of the Canon 550D and the Panasonic ZS8. Panasonic ZS8 is clearly the smaller of the two cameras. Its body is 24mm narrower, 40mm shorter and 29mm thinner than Canon 550D. As you can tell, actually this is not a fair size comparison since Panasonic ZS8 has a built-in lens but Canon 550D doesn't. Once you put a lens on 550D, 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.
Panasonic ZS8 is significantly lighter (320g ) than the Canon 550D 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 550D, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Canon EOS 550D (EOS Rebel T2i / EOS Kiss X4) nor Panasonic Lumix DMC-ZS8 (Lumix DMC-TZ18) 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 DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Canon 550D and Panasonic ZS8'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.