The most popular binocular sizes for bird watching are 8x40 and 8x42.
Binoculars with these specs are compact enough to carry with you all
day while large enough to gather plenty of light. Other features to
look for in birding binoculars are a close focus for near observation
of birds and insects, a wide field of view, enabling you to track
quick-flying birds and to see more individuals at a time when counting
or observing large flocks or groups, such as kettles of hawks, and good
depth of field so you don't need to keep adjusting the focus to keep a
small bird in focus, and don't need to turn the focus adjustment too
much to go from near to far.