Thanks for replying LB!
It is 40cm side to side (inside). I'll attach some pics with better scale capture.
But it wouldn't be hard to expand to a side, this is Mark 1... xD
I've seen some large scale producers growing directly on ground level, but I made the rised borders with a few intentions, like, as you can see weeds grow very hard here, and it would make a mess around the trees really fast, so being a little bit higher might be easier to clean the weeds into it, and keep the surroundings clean.
But you gave me an nice idea, I'm gonna do some tests on ground level too.
Other pro about the borders, is not having to dig (at least not much) when doing some root work that is required so it won't go too wild.
Thanks for the tile tip, I'll do that approach on some trees.
Here are some pics, that mix is soil, decomposing jabuticaba leaves, and rough sand, but it's not ready, gonna add some more soil, as it seems way too fluffy...