you could do
max(max([ x1 x2 x3 x4 x5 .... ]))
Since the first max is evaluated along the columns, I think it should be fast enough. Alternatively, you could of course do X = [ x1 x2 x3 ... ] and then max(X(:)). That's shorter, but depending on the length of your vectors, I would go with the first option. A common alternative is to use sort, e.g.
X_sorted = sort([ x1 x2 ... ]);
xmax = max(X_sorted(end,:))
but since for max, you don't need the matrix to be fully sorted, I would guess that max is actually faster.
