Someone gave us a stack of very large floor tile. We have a very large room
that could use a floor covering and we don't want carpet; currently it's
peeling painted cement floor. There is not enough of this tile to do the
whole room, but possibly one half of the room. But using it would probably
be a substantial savings on the cost to tile the whole room. So I was
wondering, might it be a bad idea to try to find another tile that is the
same size and perhaps make a pattern with the ones I have, or is that likely
to look bad?

Why should the tiles be the same size? (and what's "very large", 12"?)