What do you want to use the binocular for? This is very important, since it has a direct bearing on what you eventually will buy. Generally speaking, the first-time buyer should be looking for a versatile binocular. In other words, a binocular suited for watching birds, game, use on hiking and even at a sports event. It should not be too big and heavy to take on a hiking trip, or, on the other hand, too small (pocket size), which will be very handy, but which will restrict you to viewing in bright daylight (even though pocket binoculars have other benefits). As argued below, the ideal versatile pair of binoculars is one with a magnification of 8x and objective lenses of 42 mm. in diameter.
However, you could be looking for a binocular with a specific feature rather than a versatile one. Obviously, even though the manufacturers might have chosen to make a niche binocular, it will also be useful in other situations as well. If you’re looking for a pair of binocular which will be capable of handling low light situations, you could read my article on “Best low light binoculars”; maybe you’re looking for a wide angle binocular or one more suited for an marine environment, or definitely one small enough to fit into your shirt pocket, or one which can focus on an object right next to you (super close focus distance). With the exception of giant binoculars, which are too heavy and bulky to take on a hiking trip, these niche binoculars will be quite capable of handling most type of situations anyway.