  We study a transfinite iteration of the ordinal Hessenberg natural sum
obtained by taking suprema at limit stages and show that such an iterated
natural sum differs from the more usual transfinite ordinal sum only for a
finite number of iteration steps. The iterated natural sum of a sequence of
ordinals can be obtained as a "mixed sum" (in an order-theoretical sense) of
the ordinals in the sequence, in fact, it is the largest mixed sum which
satisfies a finiteness condition, relative to the ordering of the sequence. We
introduce other infinite natural sums which are invariant under permutations
and show that they all coincide in the countable case. Finally, in the last
section we use the above infinitary natural sums in order to provide a
definition of size for a well-founded tree, together with an order-theoretical
characterization in the countable case. The proof of this order-theoretical
characterization is mostly independent from the rest of this paper.
