The difference between getExternalFilesDir and getFilesDir() is clear to me and well explained here.
In my case, since I need to keep files private and not accessible by user or other Apps (at least in non rooted device), getFilesDir() seems to be perfect. However I'm going to place big files there (let's say around 2Gb in total) and I would like to know if that folder/partition is typically capable enough for that purpose (I haven't found anything about that in the documentation).
Edit:
I've found some information here. It seems that modern devices sould have at least 1GB, but I still can't figure out what is the typical partition size on modern devices...