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 in eyepieces are an older size that is still used on a few smaller telescopes. They are inexpensive to make and are well-suited to simpler eyepiece optical designs that have 2 or 3 lenses.
All NexStar SLTs use the standard size for eyepiece barrels today, the 1.25 in (1-1/4 in). 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 in size. Almost all Celestron telescopes use 1-1/4 in eyepieces.
The largest size commonly encountered is the 2 in barrel diameter eyepiece. It’s used on the 102 SLT and the 130 SLT (there is an adapter for 1-1/4 in eyepieces). The larger optics in these eyepieces give wide, unobstructed fields of view. They are heavier and more expensive than standard 1-1/4 in eyepieces and are best used on larger-aperture telescopes which have larger beams of light from the optical system.