The default configuration values are:
Backpack material: Clock.
Ender-Backpack material: Polar Bear Spawn Egg (Texture pack will override all spawn eggs)
Size: 36 (4 rows)
Both Backpack types are enabled, putting backpacks inside other backpacks is disabled by default. This setting does not apply to Ender-Backpacks, since those just open the player's ender chest.

Custom Recipe Examples

Example of current recipe:

Backpack:
shape:
- S
- E
- S
materials:
S: SHULKER_SHELL
E: ENDER_PEARL

Example for a golden apple-like recipe:

EnderBackpack:
shape:
- SSS
- SES
- SSS
materials:
S: SHULKER_SHELL
E: ENDER_CHEST