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 Pentax P80 and Canon 450D side-by-side from the front, back and top in their relative dimensions. Pentax P80 has external dimensions of 102 x 59 x 25 mm (4.02 x 2.32 x 0.98″) and weighs 125 g (0.28 lb / 4.41 oz) (including batteries). Canon 450D has external dimensions of 129 x 98 x 62 mm (5.08 x 3.86 x 2.44″) and weighs 524 g (1.16 lb / 18.48 oz) (including batteries).
Below you can see the front-view size comparison of the Pentax P80 and the Canon 450D. Pentax P80 is clearly the smaller of the two cameras. Its body is 27mm narrower, 39mm shorter and 37mm thinner than Canon 450D. As you can tell, actually this is not a fair size comparison since Pentax P80 has a built-in lens but Canon 450D doesn't. Once you put a lens on 450D, its thickness will increase significantly.
Here is the back view size comparison of the Pentax P80 and Canon 450D.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Pentax P80 is significantly lighter (399g ) than the Canon 450D 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 450D, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Pentax Optio P80 nor Canon EOS 450D (EOS Rebel XSi / EOS Kiss X2) 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
Canon 450D's 3.00" LCD screen is slightly larger than Pentax P80's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.