Get size of longest line of wrapped text.
#Redundant.
prisma crow
tame cape
You have two conflicting interests. You want a centered box, but for a box to be centered it must be smaller than its parent. How much smaller should it be? You say "the longest line of text", but that would be the whole width of the parent, because text will just flow to fill the available size if you don't constrain it.
For a centered box, you have to pick a maximum size for the box.
boreal parrot
what type of english is this? Suchka?
tame cape
Sorry, but I am not really sure I understand your problems/requirements