The most common slides are typically mounted in 2″x2″ cardboard or plastic mounts.
Odd size slides also may be in 2″x2″ mounts but have over-sized film. The exception is 110 slides in 1″x1″ mounts. Typically, a photo scanning service will mount 110 slides in 1″x1″ mounts into 2″x2″ mounts for scanning to avoid special handling fees (although the mounting often incurs a fee).
Larger size slides include medium format typically in 2 3/4″ x 2 3/4″ slides which often require specialized film scanners design for large format film.
Photo Credit / Creative Commons JTrendler