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 FZ40 and Sony A580 side-by-side from the front, back and top in their relative dimensions. Panasonic FZ40 has external dimensions of 120 x 80 x 92 mm (4.72 x 3.15 x 3.62″) and weighs 494 g (1.09 lb / 17.43 oz) (including batteries). Sony A580 has external dimensions of 137 x 104 x 84 mm (5.39 x 4.09 x 3.31″) and weighs 599 g (1.32 lb / 21.13 oz) (including batteries).
Below you can see the front view size comparison of Panasonic FZ40 and Sony A580. Panasonic FZ40 is 17mm narrower and 24mm shorter than Sony A580 but it is also 8mm thicker. As you can tell, actually this is not a fair size comparison since Panasonic FZ40 has a built-in lens but Sony A580 doesn't. Once you put a lens on A580, its thickness will increase significantly.
Here is the back view size comparison of Panasonic FZ40 and Sony A580.
Now lets look at the top view comparison of Panasonic FZ40 and Sony A580.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Panasonic FZ40 is significantly lighter (105g ) than the Sony A580 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 A580, the total kit will weigh much higher.
Unfortunately neither Panasonic Lumix DMC-FZ40 (Lumix DMC-FZ45) nor Sony Alpha DSLR-A580 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 Bridge Cameras with Weather Sealing Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Panasonic FZ40 and Sony A580's LCD screens has the same diagonal size of 3".
On the other hand, Sony A580 features a Tilting screen
where you can change the angle of the screen to make it easier to shoot from waist or over the head levels whereas the Panasonic FZ40 has a fixed type screen which provides less flexibility in shooting positions compared to A580.