ideal cluster size

[Thorsten von Eicken]

We're embarking on a project where we estimate we will need on the order
of 100 cassandra nodes. The data set is perfectly partitionable, meaning
we have no queries that need to have access to all the data at once. We
expect to run with RF=2 or =3. Is there some notion of ideal cluster
size? Or perhaps asked differently, would it be easier to run one large
cluster or would it be easier to run a bunch of, say, 16 node clusters?
Everything we've done to date has fit into 4-5 node clusters.