How to determine the SPIFFS size on an esp8266-01?
First you need to know the size of the flash memory attached to esp8266 on your module. Common sizes are 512 kB (4Mbit), 1 MB (8Mbit) and 4 MB. Espressif Flash Download Tool (Windows) or esptool can determine it.

You can choose the size of the SPIFFS in your flash 'layout' by selecting the option in IDE Tools menu. Take one according to for you flash size and size of the files you want to put into SPIFFS.

If you have 1 MB flash on your module and you want to use OTA upload, let flash space for twice a space taken by the sketch.