I can't find the size of a page of memory in an STM32L0 (specifically STM32L053R8, see DocID 025844). I don't see a referenced as to the size of a page of memory.
Note: This is also in reference to the native boot-loader.
You can find this information in the reference manual at page 50 (chapter 3 - Flash program memory and data EEPROM).
There is the actual organization of the non volatile memory.
For this device a page of flash is 128 Bytes.