This method, which is used in the example of iterating through EEPROM memory, is not documented.

In eeprom.h version 2.0

This function returns an unsigned int containing the number of cells in the EEPROM.

As shown in the library file

uint16_t length(){ return E2END + 1; }

E2END is an AVR macro defined at <avr/io.h>: AVR device-specific IO definitions

The last EEPROM address.