Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Panasonic ZS1 and Sony A550 side-by-side from the front, back and top in their relative dimensions. Panasonic ZS1 has external dimensions of 103 x 60 x 33 mm (4.06 x 2.36 x 1.3″) and weighs 229 g (0.50 lb / 8.08 oz) (including batteries). Sony A550 has external dimensions of 137 x 104 x 84 mm (5.39 x 4.09 x 3.31″) and weighs 632 g (1.39 lb / 22.29 oz) (including batteries).
Below you can see the front view size comparison of Panasonic ZS1 and Sony A550. Panasonic ZS1 is clearly the smaller of the two cameras. Its body is 34mm narrower, 44mm shorter and 51mm thinner than Sony A550. As you can tell, actually this is not a fair size comparison since Panasonic ZS1 has a built-in lens but Sony A550 doesn't. Once you put a lens on A550, its thickness will increase significantly.
Here is the back view size comparison of Panasonic ZS1 and Sony A550.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Panasonic ZS1 is significantly lighter (403g ) than the Sony A550 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 A550, the total kit will weigh much higher.
Unfortunately neither Panasonic Lumix DMC-ZS1 (Lumix DMC-TZ6) nor Sony Alpha DSLR-A550 provides any type of weather sealing in their body, 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
Sony A550's 3" LCD screen is slightly larger than Panasonic ZS1's 2.7" screen.
In addition to the size advantage, Sony A550 LCD screen also has a Tilting feature
where you can change the angle of the screen to make it easier to shoot from waist or over the head levels. On the other hand, Panasonic ZS1 has a fixed type screen which provides less flexibility in shooting positions compared to A550.