Hi there
ZEsarUX already includes its own SD image, you can look for tbblue.mmc file. Also, you can download official images just directly from the menu, see:
https://www.youtube.com/watch?v=2a1Kv8HiP5M
I don't recommend any image larger than 1GB, but it's up to you. Also, starting from 2 GB, seems to have problems reading those image and/or showing the file size (your negative numbers)
Cheers