I couldn't offhand find any photos that truly show the owl's size. But there's this page with photo (and you can email them as postcards.
There's this page
It also includes a Real Audio sound recording:
Then there's this on Harry Potter's owls which notes:
Considering the flaunting of wealth and display of arrogance, it isn’t surprising that the Malfoy family bird is the Eurasian eagle owl, Bubo bubo. This raptor is one of the largest of the European owl species and is an average of 22.8 to 28 inches tall. The feathers on the underside of the owl are brown-black and tawny, with a freckling effect on the head; stripes on the neck; and splotches on the back and shoulders. The owls has tall ear tufts and an upside-down triangular patch of white under its chin, ending in white on the upper chest.
Females range from 5 to nearly 10 pounds (2,280-4,200 grams), and males, 3.5 to 6.6 pounds (1,620-3,000 grams). They will capture prey either from a sitting position on the ground or swooping down from the air. The owl’s diet include many small