I believe you are referring to Trisquel's graphical installer, which does not
propose full-disk encryption (only encryption of the home folder with
eCryptfs, whereas GNUbahn is talking about a the text installer. I believe
he follows the instructions on
https://trisquel.info/en/wiki/full-disk-encryption-install
Anyway, the text installer has the equivalent of "Do something else": it is
called "Manual" according to a screenshot in the manual linked above.
XFS is said to be faster with large files, hence Trisquel's default for
/home. A few MB is "large" in this context. Its largest drawback is: it
cannot be shrunk. So, if you under-size an adjacent partition, it will not
be easy to take space from the partition with XFS: you would have to copy its
data elsewhere, delete it, extend the adjacent partition, recreate a
(smaller) partition with XFS (or not, if you change your mind) and copy the
data back onto this new partition. So, well, you had better not under-size
the adjacent partition.
