Recomended storage size for Zigbee2mqtt

I want to buy a SSD for Zigbee2mqtt to run on my Pi3B+.
However I am not able to find anywhere what the recomended size would be.
I don´t want it bigger than necassary because I like to keep images stored for backup.
I would think 16 or 32Gb.
Anyone knows what would be ideal?

I use a 16GB SD card on a Pi2B

thanks for answering