Sizes 101

The chart above shows the dimensions of the notebook while is is laying flat open,  from edge to edge.
 Overall, my notebooks are slightly wider than the common, allowing for no overhang if you follow these guidelines. Your notebook edges will be fully protected.
Below is the information for the corresponding inserts.   These measurements are the sizes of the inserts. 
Pocket  fits notebooks approximately 3.5” x 5.5”. Most commonly used are Field Notes or Moleskine inserts. Up to 5 inserts less than 1” thick  
A6 fits notebooks that are approximately 4"x6" (i.e. The Hobonichi Techo),  And do not exceed more than 1.5 inches in depth
Personal fit inserts that are  3.75" x 6.5 and do not exceed more than 1 inch in depth.
Narrow will hold up to 5 standard Midori brand inserts (approximately 4.5” x 8.25”) that are not more than 1” thick.
Wide  is sized for 5” x 8.25” inserts (usually Moleskine Cahier sold in a three pack). At least 4 inserts that do not exceed 1” thick will fit.
A5 notebooks that measure "5.8x8.25" (i.e. the Hobonichi Cousin) and do not exceed more than 1.5 inches in depth.
 If you are not sure what size to get for your inserts.  Measure them carefully and find the size that is the same or similar.  Then stack them up and measure how thick your inserts are.  Then you can see if you are using 4.5" x 8.25" inserts that are about 1 inch high,  you'd know you need the Narrow size.  But if you have a really chunky notebook and have more than 1 inch then you'd want to go up to the Wide.