If you want to use any of the pins on the 2x4 connector as I2C interface, you need pull-up on those pins. ESP-01 module doesn't have pull-up on-module. We have to connect the pull-up on the mating board.
The value of the pull-up resistors on I2C interface vary depending on the I2C device connected, I2C device operating voltage, rise time of the I2C signals (get from datasheet), power consumption constraints, PCB trace routing (capacitance), speed at which we are operating I2C interface.
ESP8266 supports both Master and Slave configuration and maximum operating frequency of 100 kHz.
Refer this note to how to calculate the I2C PU resistor value
Even there are some calculators available which help calculate the pull-up resistor value after entering the required electrical requriements.
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