Below are 4 non-standard photo slides sizes (or types):

 

  1. 2.75 inch by 2.75 inch cardboard mount slide.
  2. 2 inch by 2 inch thick metal mount glass slides.
  3. 1.25 inch by 1.25 inch plastic mount slides.
  4. 2 inch by 2 inch cardboard mount with additional metal mount.

Click to enlarge.

You can view different

2x2 photo slide film formats HERE.

2x2 Photo Slide with Additional Metal Mount

Click to enlarge.