Maybe I'm wrong but this is what I got from your link
I should create an txt file with this content following the folders
/recalbox/share/roms/mame/blazstar.zip.recalbox.conf
The game name is just an example, what I don't get it what I should write on recalbox.conf
PS: found some instructions on github
video_aspect_ratio =
I guess that I should write this to get what I need
video_aspect_ratio = 1:1
can you confirm this? @zing