Standard glass slide: 3¼ inches (83mm) by 4 inches (100mm)
Sample image of this type of slide – slide shown is slide 31 of Das Wunder Aziens: Teil I (lecture: Franz Stoedtner, 40 slides, in/after 1895)
EYE Film Institute Netherlands – reproduced by permission. Digital image © 2016 Sarah Dellmann – CC0 (no copyright).