I don't know what you need that Preferences thing for, but I would use SPIFFS/FAT anyway. Simply store the certificate in a file in flash and read that file at when you need it. The required space is no problem as you can choose the size for file system in the partition table and also you can upload the file (there is a plugin in the arduino IDE for this purpose) after flashing the firmware, so having different certificates on individual devices is no problem.

I saw that you tagged the question with 'eeprom': the esp has no eeprom but emulates it with a blob in flash, so using any flash library is just as fine. The esp also supports wear-leveling, which makes using flash an even better option as the dedicated eeprom-partition, which was used in an older version of the sdk, would wear out orherwise. The "eeprom" is just there to be consistent with the use of other arduino's eeproms.

I don't know what you need that Preferences thing for, but I would use SPIFFS/FAT anyway. Simply store the certificate in a file in flash and read that file when you need it. The required space is no problem as you can choose the size for file system in the partition table and also you can upload the file (there is a plugin in the arduino IDE for this purpose) after flashing the firmware, so having different certificates on individual devices is no problem.

I saw that you tagged the question with 'eeprom': the esp has no eeprom but emulates it with a blob in flash, so using any flash library is just as fine. The esp also supports wear-leveling, which makes using flash an even better option as the dedicated eeprom-partition, which was used in an older version of the sdk, would wear out orherwise. The "eeprom" is just there to be consistent with the use of other arduino's eeproms.