Stuck on which size to get? A lot of the cover sizes will allow you to easily buy inserts at the local stationery/business supplies store, or online (there are lots of Etsy shops that stock inserts - just search "traveler's notebook inserts" and look for a size that corresponds to your leather cover). Some sizes are compatible with Moleskin, Field Notes, and Leuchtturm1917.
Here's a visual for you comparing all sizes to a 12 inch vinyl, a 45 (which is 7 inches), and a paper back novel.
In this picture:
Dark brown book is B6 size
Light brown is Pocket size
Black is Narrow size
Grey is A5 size
Reddish brown is Wide size
Dimensions + insert sizes
Wide cover dimensions when folded: 5.75" x 8.5" (14.6 cm x 21.6 cm) *Compatible with Moleskin Cahier Large, Field Notes, and Leuchtturm1917 or any insert sized 5" x 8.25" (12.7 cm x 20.95 cm)
Narrow cover dimensions when folded: 5.25" x 8.5" (13.3 cm x 21.6 cm)
*Compatible with midori inserts found online or any insert sized 4.33" x 8.25" (10.99 cm x 20.95 cm)
B6 cover dimensions when folded: 5.75" x 7.5" (14.6 cm x 19.05 cm)
*Common notebook size in stationery stores. Compatible with any insert sized 5" x 7" (12.7 cm x 17.78 cm)
A5 cover dimensions when folded: 6.75" x 8.25" (17.15 cm x 20.95 cm)
*Common notebook size in stationery stores, compatible with Leuchttrum1917 and Rhodia, or any insert sized 5.8" x 8.25" (14.7 cm x 20.9 cm)
Pocket cover dimensions when folded: 4.25" x 6" (10.8 cm x 15.24 cm)
*Compatible with Moleskin cahier pocket, and Field Notes or any insert sized 3.5" x 5.5" (8.9 cm x 13.97 cm)
If you're already all moved in to a TN and you have inserts already please consider the following:
Measure your existing inserts and find the size that is closest. Note if you're between sizes, you should pick the larger size - you can always fit smaller inserts in to a larger size!