ESP8266 Size can be anywhere between 4 and 4096 bytes.
ESP32 Size can be anywhere between 4 and 1984 bytes.
The IAS library uses EEPROM for storing the following things:
And if you are using the addfield() method these fields are also stored in EEPROM in a array structure.
The maximum nr of fields in this array is defined with MAXNUMEXTRAFIELDS in the config.h file.
As the IAS library uses EEROM and the function .end() You are advised to do your EEPROM "stuff" after IAS.begin(); especially if you are going to use it in a global scope.
If for whatever reason you need to put your EEPROM code above the IAS.begin() keep in mind that our libary will .end() it...
From where can I use EEPROM?
This depends on how many fields you have added to your app and how big these fields are. Since 2.0.1 you can use the following var to query from where it is safe to use eeprom. Anything after that is yours to use.
IAS.eepFreeFrom; Serial.print(IAS.eepFreeFrom); // will return a unsigned int eg. 536
The serial monitor will also show you the locations in EEPROM.