Nor Flash File System with create variable amount of data file

Question

I had interfaced with STM32l0 Controller with my external serial nor flash MX25R in order to save a log file. Everything works fine during the operation like creating file, Writing and reading data. But my problem is that i am able to create a certain file size like 1024 for example: SerialFlash.create(SF_Name, 1024);_SF = SerialFlash.open(SF_Name); I would like to create file like with variable amount of data as my file size may vary according to user setting. So may be some time 1 mb to 10 mb also. So how do i create this way of implementation? Thank you.