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 Panasonic ZS3 and Sony A390 side-by-side from the front, back and top in their relative dimensions. Panasonic ZS3 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 A390 has external dimensions of 128 x 97 x 86 mm (5.04 x 3.82 x 3.39″) and weighs 549 g (1.21 lb / 19.37 oz) (including batteries).
Below you can see the front-view size comparison of the Panasonic ZS3 and the Sony A390. Panasonic ZS3 is clearly the smaller of the two cameras. Its body is 25mm narrower, 37mm shorter and 53mm thinner than Sony A390. As you can tell, actually this is not a fair size comparison since Panasonic ZS3 has a built-in lens but Sony A390 doesn't. Once you put a lens on A390, its thickness will increase significantly.
Here is the back view size comparison of the Panasonic ZS3 and Sony A390.
Now lets look at the top view comparison of Panasonic ZS3 and Sony A390.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Panasonic ZS3 is significantly lighter (320g ) than the Sony A390 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 A390, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Panasonic Lumix DMC-ZS3 (Lumix DMC-TZ7) nor Sony Alpha DSLR-A390 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 DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Panasonic ZS3's 3.00" LCD screen is slightly larger than Sony A390's 2.7 screen.