Astronomical eyepieces are described by the diameter of the base tube or barrel and come in several standard diameters. This is not to be confused with the focal length of individual eyepieces. The eyepiece diameter for your scope is important to know when purchasing other accessories, too, such as cameras, adapters and filters.
0.96-inch eyepieces are an older size that is still used on a few smaller telescopes such as the PowerSeeker 50AZ. They are inexpensive to make and are well-suited to simpler eyepiece optical designs that have 2 or 3 lenses.
The standard size for eyepiece barrels today is the 1.25-inch or 1-1/4-inch. Most PowerSeekers use this size of eyepiece. There are many different designs and focal lengths available from many manufacturers. Almost all accessories that fit into focusers are available in the 1-1/4-inch size. Almost all Celestron telescopes use 1-1/4-inch eyepieces.
The largest size commonly encountered is the 2-inch barrel diameter eyepiece. The larger optics in these eyepieces give wide, unobstructed fields of view. They are heavier and more expensive than standard 1-1/4-inch eyepieces and are best used on larger-aperture telescopes which have larger beams of light from the optical system.